Hi Hogy,
The above answers are right on. I would add to Kerry's point 3 that it is more likely that the axolotl you have will be larger than any newcomers, so that the 1/3 difference in size means that the new tank mates should not be 1/3 smaller than the one you already have (or your existing pet may try, more or less successfully, to eat the newcomers).
Axolotls are not territorial per se, but the do get large as adults and do need room.
As Oregon Newt wrote, the single most common and serious problem that new axolotl keepers experience stems from problems with water quality. Axolotls produce a surprising amount of waste (ammonia) which needs to be remedied by bacteria, which need time to develop.
Follow the links Oregon Newt included and you should be well armed (informed).
In any case, welcome to the addictive world of axolotls!
-Eva