We are running RotRL as well and I am playing an Aasimar Paladin in the campaign. Our GM doesn't have any problems with it, actually I had some nice RP moments because of my race.
What I did , I gave my GM some infos about Aasimar from the "Blood of Angels" so that he got an idea about how other people see them and so on.
Most of the races in the arg are pretty much fine. Some a bit stronger then some of the core races, but other then drow noble, none of them are game changers.
Most of the time its a matter of the capmaign and whether or not you want all sorts of crazy creatures in your party or not, rather then a matter of power. I would keep an eye on him, but it isnt explicately a problem.
That said, as a beggining gm, its usually a good idea to keep things simple at first. You might for instance want to run a shorter campaign (like say dragons demand) first as a core only game, get a feel for how running a game is, and then open up some options and start rise of the runelords (which is a much bigger investment of time and effort on everyones part).
They wouldn't be surprisingly overpowered. Maybe have an extra point of ac here or there or an obscure spell like ability. Generally the biggest thing about those two races is that they are not humanoid so they are immune to the -person- line of spells like hold person and charm person. They also would take double smite damage from smite good or smite evil. Beyond that as long as it's not a custom race it's not going to blow up the campaign.
http://www.d20pfsrd.com/races/other-races/featured-races/arg-aasimar
Above is the link to the pfsrd on aasimar's you can read all about their abilities there. The thing that stands out most is the low level elemental resistances. Your player will probably go for angel blooded aasimar, which will give them a minute of +2 to strength a day.
It's a bent race, probably one of the strongest races allowed to pc's. But it isn't going to make him stronger then everyone else by a large margin. To compensate I would at least allow the other players the options to use alternate racial traits listed on the SRD.
They won't take the double damage from Smite, as both are Native subtype, not Good nor Evil subtype.
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Oreads are less powerful than Aasimars. They Make really good Monks and Melee Clerics, and are a flavorful choice for Elemental Earth Sorcerers.
Some of them are a bit more powerful than the CRB races, while others are fine. Even so, there is the fact that more options means things will be more powerful, as there will be greater chance for a certain class-race combination to be overpowered, as well as the races themselves to go out more standard deviations from the mean, in terms of power.
Things I'd look out for: Aasimars & Tieflings, especially when used to qualify for PrCs. Any race that is between 12-20 Race points. Any race with spell resistance or anything similar. Anything that seems like it's chosen simply for the mechanical benefit* (These are things to look out for and be cautious of, but not necessarily ban. Though I'd err on the side of banning, if you're not sure; also *Not that it's wrong, mind you, to build mechanics first, but it could be a sign of more min-maxing than a new DM can usually handle)
Things I'd ban outright: Anything with 20+ Race points. Anything who has a racial feature that could really screw up your game (such as certain SLAs or abilities such as Flight)
I would recommend anyone who is new to GMing not allow custom races. and anyone who does allow custom races let the gm build them not the player. i have seen players build their own races and this become a problem in game so i would warn away from that for now.
Since you are relativly new to the game i suggest you keep the rules to books you have and have read. The game is huge and it is no fun for a GM to navigate it all because he have players with internet.
I think, for exampel, the Oread sorcerer is gonna take the wildblooded Archtype that allow wis as casting stat.
The different kinds of asimars is not from the ARG but from some pamphlet but the rules, good and not so good, can be found in the SRD.
I find important as a GM to decide from the start what books we use and then stick to it. That way i will have a chance to know the rules, and the obnoxious guy, that never stop asking to play somthing from somthing else, wont get special treatment. I set these limits at the same time as i talk expectations and genre with the group.
I'd ask for a basic build plan. If he wants an Aasimar oracle with the Heavens mystery and the golden angelic skin feat, then I'd say no.
The game doesn't break if you allow someone to play a non-CRB race and it doesn't take vast experience to GM for someone playing a non-CRB race. There's a few races that are sufficiently far outside the norm that you'd have to adjust as a GM (merfolk and strix, mostly), but from what you said, your player isn't asking to play ony of those races. The absolute most impact this would have is your player's character getting access to some abilities (for example, energy resistance) a few levels earlier than normal.
Do the requested races in and of themselves break the game? No. Do they open up a potentially big can of worms for a new GM? Possibly.
The problem is that as a new GM, you are no doubt going to be learning a lot as you go (I've been GM'ing for over 15 years and I am still learning). Typically, in my experience, adding one extra race like that won't be a game breaker. However, what happens when one of the other players decides he doesn't like his dwarf character and wants to pick another race out of the ARG? It's going to be hard to tell that player "No." when you've already allowed one race. Next thing you know, you have a full party of non-standard races and you're being asked to consider how a Magnimar citizen is going to react to a Bugbear that is by all appearances good aligned, etc.
Personally, my recommendation would be to keep it simple. Stick with the Core Book for races and explain to your player(s) that while you doubt the character concept will be game-breaking, you are afraid that while you're still learning how to GM, the additional races might be a bit too much for you to handle at this time.
As a side note, this might be a particularly good idea since most of the adventure paths are not designed to be overly difficult, though obviously different groups will have different experiences.
So, I'm also a new GM, and to get more used to the game and avoid similar problems, I've done a themed game: All-dwarf. It allows me to get a better hold on the game, and those interested wont get distracted (since they knew from the beginning it's all-dwarf).
From my own experience playing CUSTOM races (a GM I play for likes to mess around, so we used the race creation guidelines) can be messed up, but not unpleasant. Everyone's got a custom race, so we're all having fun. But that's not something I will do soon, and not something to reccomend to a beginner).
Don't let him be a Duergar, they can turn invisible once per day. Granted so can wizards and such, but not off the bat!