You can use any gauge that you're able to draw thru.. The skinnier the needle the better, since you get less scar tissue.. 22-23g is generally viewed as the standard for gear, but I'm using 25g this time around.. I don't mind if it takes a little longer to draw/inject..
As far as length, 1" should be OK, just make sure you're burying that thing all the way in.. When I use 1.5", I don't usually go ALL the way in with a glute injection, so I don't think 1" would be a problem.. I've heard of some guys going even shorter for delt injections - something like a 5/8" I think it was.. You just wanna make sure you're getting that oil right in the middle of a nice chunk of muscle, and not near the skin or anything.