Simple Q&A Pathfinder (By RAW) I

[Larpus]

A498
No.

Usually, when it says "you need to be level X" they actually mean "you need to be level X in this class". There might be a few exceptions, but this is not one of them.

A499
Not really, all that would happen is that someone would be able to mount you in battle and use mount-related abilities and maneuvers.

If you want a centaur, all you have to do is grab the Limbs(arms) evolution on a quad eidolon as there are no rules discriminating how long is your eidolon's torso and how is it shaped (you still wouldn't get the extra 5ft in range when enlarged though).

But if you're interested in making a centaur down to the mechanics, ask your DM if you'd be able to do it with Mount+Limbs(arms).