My first axolotl was just under 4 inches when I got him and 3 months old. He's grown an inch in the time I've had him (almost a month). I just picked up another one at 5 inches this weekend and they are housed together in the same tank. I have a 20 gal one, but aim to get a larger one in the next couple of months when I have the funds.
They've only been together a couple of days, but they haven't nipped at each other yet as far as I can tell. They have 2 hides each to themselves and pretty much ignore each other. As ACEPHONECHIC said, as long as they have enough room, hides, and food, they should be fine together.