Hi,
Yes 2 foot is fine for one axolotl i have one in a 2 foot tank. But i warn you they are addictive i have had mine for about 5 months and im already looking for a 3 foot so i can get another!
Yes marimo moss balls are fine. I tend to buy plants from an aquatic store rather than ebay though as they seem to be better quality and you can see how healthy they are. Java moss and Anubias plants seem to do well in axolotl tanks too.
I use a fluval u4 filter?i think that's what it is!its an in tank filter,the ones out of the tank/canister ones seem to come highly recommended,I'd say buy the best filter you can do that if you end up getting a bigger tank you can move the filter over and it still do its job.
I might be talking rubbish it's been a long day but when I went to buy mine the guy said if I ever got a bigger tank (mines 220l) I'd need a canister filter because the one I got only does up to 230l.someone else might be able to explain better what I'm on about!
I use a sponge filter with an air pump attached, and i know it's not recommended but I have two in a 2 footer and they are perfectly happy...
They seem to work great!i forgot about the sponge ones!