[Dwarf Mexican Crayfish Ideas?]

Crayfish can and will climb on/in anything. Since you aren't going for breeding or providing hides from predatory fish you can pick whatever you like. I'm a huge fan of broken clay pots but you can choose whatever. 

[Air pump size suggestion for 7 sponge filters]

Not sure of what size to use, but one thing to keep in mind is that the length of tubing will make a huge difference. In my experience I have been able to run 3 filters off of a medium sized pump if the tubes are short enough, but put that same pump up to my 40 gallon where it has to sit on the floor and it struggles with the one filter.