Mmm, I don't know with Fred Bare. DS only has a few things, the tops seem true to size, but the pants seem quite small to me. He's already wearing size 0 FB pants, but is still in 00 in most other things.

And we were given some FB pants, size 1, and when I held them up to his size 0 pants in other brands, they were about the same.

Sorry, that's probably not much help! But I definitely think that they might be a little on the small side.

As for your sizing question, my DS is average size for 9 months, and is still pretty much in 00 all round. Unless he has a massive growth spurt soon, I don't see him being a true size 0 until closer to his 1st birthday.