I looked at getting axolotls from them and they also wanted me to pay $250 in shipping (they have a canadian location, too) which is such a rip off.
As for exactly when they're out of their cannibalistic stage, I think it all depends on the individual. In my case, there was plenty opportunity for them to be cannibalistic, but since they were born, nothing. As long as you kept them well-fed and with hiding spots they should be fine. I had a couple of people take some home when they were 2.5" in groups of 3, and they are all doing fine, no injuries to speak of.
The one possible concern I can think of is how far along has Mjoll healed with his/her injury? Axolotls that have large injuries like that tend to have stunted growth for a while because their efforts and energy is all put towards regeneration, rather than growth. If it's not fully healed I would wonder whether or not the new axolotls will grow faster than Mjoll and out-size him or her. However if it's mostly healed, you'll probably be fine. This is based of what I'd consider common sense, but I've never had an injured axolotl so perhaps someone can confirm.
Just make sure when you upgrade, footprint is more important than height. For instance, 30"x12" I would consider being bare minimum for 2, and even then that may push it depending on the size the axololts get (they won't grow according to tank size like some people think). They can get as big as 17" long (10-12 being average) so you can imagine how squishy that would be in a 30"x12".
For 3, I would suggest getting something that has a 42"x12" footprint. Again, 36"x12" may work as a bare minimum, but that really depends on how big your axies grow. The bigger footprint you can get, the better. You may also have people in the area that can custom build tanks for you. I got my 60"x15"x15" built for $130, then I just bought a light to go on top. Perhaps you can find a used 55gal tank on craiglist? That should work nicely for 3-4. As the tank size gets bigger, it's more of a concern of water quality rather than footprint, as all axolotls will have access to that larger footprint. I currently have 6 in my big 60" tank, however I plan on adding 3-4 more and just being diligent with more frequent water changes. Now that my sub adults have been put together they actually seem to enjoy having each other for company, snuggling up in their DIY hide I made for them.
Again, this is all my opinion, others may feel differently.
