Sounds awesome! I've also never done clams, but my two cents:
Remember that since this a cube you will have much less surface area at the top than you would on a more standardly proportioned 25g tank. That means you will need to stock it more lightly than a more standard sized tank, or filter the heck out of it, to be sure that your water is well oxygenated, because you won't get as much air exchange at the surface as you usually would.
I'd also be careful about stocking to be sure you don't put something that likes a lot of swimming space in it for the same reason. Fish like zebra danios, that spend their time doing laps around the top of the tank all day, have much less "top" to chase each other in, than in a tank with the same volume, but rectangular shape. So you'd be able to stock fewer active fish, or you'd start getting competition for territory.
Anything else going in besides clams? So cool!