Plants, plants, PLANTS! STUFF that thing! If you think there are enough, add more! Add as many hiding places as possible too. I currently have 6 girls in my 10 gallon and soon to be 8- I didn't slowly put them in, but I did QT every single one for 14 days and then added them in all at once. I've added additional girls to the mix a few times, and I've never had a problem. Congrats on considering one, they're so much fun!
55 Gallon: Crowley: Melanoid Axolotl; Mischa: Melanoid Axolotl; Achlys: Albino Axolotl; Eddard Stark: Leucistic Axolotl
Furry Friends: Daisy and Holly: Dogs; Norman and August: Cats; Gilbert and Mordecai: Guinea Pigs