If they are similar in size you can keep them together. Ideally you have more hides than axolotls. I would say they are like goldfish in that you should have at least a 20long for one, and an additional ten gallons per additional axolotl. A larger footprint is obviously better than height.
Wild type, white albino, gold albino, leucistic, black melanoid, and grey melanoid are all easy to come by. I got mine from a breeder based in PA who will ship, has great pricing, and healthy stock. The one I wish I could find (and so does the breeder) is the piebald. I have seen pics of them and they are amazing, they are white with black blotches.
There are five very important things when it comes to axolotls:
1-Temp has to be kept low. They are true coolwater animals and will be stressed if kept at temps over about 73F long term.
2-High quality food is vital. Fortunately almost all axolotls are raised on pellets and have no issues taking to them. I feed mine New Life Spectrum exclusively and the growth matched that of the breeder.
3-Substrate is more important that with fish. Gravel is a no no. In addition to the normal trapping debris which lowers water quality, they can ingest the gravel which can cause an impaction. Bare bottom stresses them by not having anything to grip and can cause sores on their toes. Sand is ideal If ingested it can pass through without issues, provides something for them to grip, and won't trap debris.
4-Aeration is important. It increases oxygen levels and can make a huge difference if the temp ever does get toward the high end of their range (can be what saves them in a bad situation).
5-Water quality is just as important as it is with fish. Many people lack in this because they are not fish, but water quality is just as important with axolotls as it is with fish.
In general there are no safe tankmates for axolotls. Either the axolotls will eat them or their gills will be nipped by the fish. The only thing that has worked for me is fancy goldfish, they are too slow to nip the axolotls without getting nipped themselves and are too big for the axolotls to eat.