I usually fill my tanks fully when I feel my juvies are strong enough swimmers (when their legs are fully grown and they're flitting around freely). This occured when my youngest were roughly 4 inches. I think it's more of a preference to be honest, when you feel they've grown slightly, add a little more water.
As filters go I know a lot of people recommend sponge filters. Having said that I've always used under gravel filters and never had any problems with them other than finding suitable substrate - can't use sand with under gravel, so large rocks it is! I don't think it matters all that much so long as the current isn't affecting the axolotl and you feel it's doing its job by keeping your water clean! :happy: