Technically, you are supposed to have already chosen your archetype when you took the first level of Ranger. If you GM lets this slide, however, there is nothing preventing you from being a Sable Company Marine. Archetypes rarely have additional requirements, and this one is no exception (though a GM might require you actually be a member of the Sable Company if you are playing in Golarion). Any Ranger would be free to take the archetype, provided they haven't already taken an archetype that alters or removes Hunter's Bond or the first Favored Terrain.
I would think you don't get the Hippogriff until level 4, since their "Hippogriff Companion" ability specifically states "This ability works identically to hunter's bond when used to gain an animal companion." However it isn't explicitly stated in the write-up.
Apart from that, i seriosly doubt you GM will just give you a Hippogriff companion, no matter what. If you talk to him, maybe he´ll be able/wanting to weave something along this line into his campaign, but the Sable Company is pretty regional specific and absolute elite.
Well archetypes do not have to be chosen until the first level it changes something.
Are you sure? That isn't how I have been reading it.
Most of the options presented on the following pages include a host of alternate class features. When a character selects a class, he must choose to use the standard class features found in the Core Rulebook or those listed in one of the archetypes presented here.
Now for rangers that tends to be level 1 as it alters your class skills..but not all do.
I can think of three that alter class skills: Beastmaster, Trapper, Urban Ranger. Out of 18, I'm not sure you can say they "tend" to alter class skills.