I don't have any experience with sponge filters, but I have noticed that others use them, so you could always have a look at the forum section specifically on axolotl tanks, set ups and filters, and read through some of the previous posts to see if you can find some posts specifically relating to sponge filters.
I'm quite new to axolotl keeping, and (as I mentioned) I don't have those filters, but going through the other posts might save you from waiting for a more experienced keeper to respond.
As for tank size, I have 3 axolotls in a 200L (approx. 50 G) tank. No reason why you can't move yours to a larger tank as they do like to have a walk around. Flor space is more important than depth of water.
I hope that helps.