You'll be constantly upgrading as you go. If your GM allows blood money and your Necromancer has a decent Strength, you'll be fine. Even if not, just prepare to budget for onyx accordingly. Since your first ability to animate dead is from lesser animate dead under normal circumstances, you might as well not learn that one and wait until 7th level when you get animate dead. At that juncture, you're looking to animate the biggest corpse you come across.
Since you're limited to one "pet", you want to look at the available variants to use up as much of your animation limit ('bucket' according to one well-written guide on this very topic).
I highly recommend preparing the stat card for your "pet" as soon as you animate the corpse. If your campaign involves numerous trips into humanoid towns/cities/et al, prepare a way to stash it. This can be as simple as a cave or something else similarly out of the way within, say, dimension door distance of the town gates.
With the limits in place, preparation is more vital than ever. Six levels will pass prior to acquiring the spell, so setting aside money over those levels for your supply of onyx is highly recommended. 50 gp here and there over those levels will add up surprisingly fast. Be sure to acquire a handy haversack fairly quickly.
It's viable. PFS uses a similar rule as I understand it, and I also have a similar rule for my home games.
Too many creatures bogs down combat turns leaving other players bored while you take your turn and can also give "your character" too much "power".
Regardless, it's completely viable to have a single powerful undead following you around. I highly suggest you use the variant bloody skeleton to make sure it sticks around with you forever. Just keep animating the biggest thing you can get a hold of. If your campaign doesn't have you meeting new interesting big enemies that are worth killing to animate, ask your GM if you can go find one for that exact purpose. If you're going to invest in it you should have a chance to use it.
Command Undead is for taking over someone else's zombies, basically. It has its uses, and it certainly fills in the long wait until you gain access to the spell at ~13th level.
Since animate dead is already on your spell list, UMD is the hardest way to use a scroll. You can already attempt a caster level check against a fairly low DC to use it yourself if the only thing you're lacking is the ability to cast animate dead.
Depending on what you animate, it comes with certain proficiencies in armor and weapons: giants are awesome for this. Strap it into the best suit of armor it can wear without penalty, assuming it didn't die in a suitable set of armor. Depending on its proficiencies, give it its old weapon(s) and perhaps a heavy shield. Zombies are prone to decay, so a sihedron medallion keeps it from stinking up the place and attracting carrion eaters.
Some of the usual buff spells won't be of any use - anything applying a morale bonus doesn't work nor do any mind-affecting buffs. haste gets great mileage and gives you something else to target besides your adventuring buddies. Coordinate with your fellow adventurers on their buff spells - barkskin, ironskin, magic fang or magic weapon and its cousins, magic vestment especially on its armor and/or shield.
If need be, set aside gp to acquire CL 12th wands of Extended magic vestment (SL 4th), barkskin (SL 2nd) and Extended greater magic fang (SL 4th) or Extended greater magic weapon (SL 4th) as appropriate. You're really going to want to have the nastiest wands of inflict wounds you can get your grubby paws on to keep your undead pet unlive and kicking after a fight since they don't naturally heal hp. The Extended wands above are for poking your zombie/skeleton 1/day for each benefit, giving 24 hours' of +3 armor AC, +3 shield AC and +3 enhancement bonus to the targeted weapon or natural weapon. barkskin at 12th provides another +5 natural armor for 2 hours, so an Extended wand of barkskin is SL 3rd - but it lasts 4 hours instead of 2. Use it first, before haste, unless a party member is willing to provide barkskin or haste.
Finally, be prepared to dispatch your own minion if something else compromises your control.
Keep in mind, animated targets lose all HD from class levels, only racial HD remain.
So giants make decent choices because they tend to have a good amount of HD for their CR, and decent powers. Humans and most other humanoids are bad choices since they don't have racial HD, and animating a 1 HD creature is going to be incredibly weak no matter how you buff it.