Ive kept clams in a 20g before. Dont see any problems there. Mainly, you have to have a good light, clean water and regularly feed the tank with coral foods. But, in order to do it right, you have to do your research and learn how the tanks chemistry will effect all your critters.
For one thing, i would ditch the canister filter and buy a ten gallon tank for a sump and use a sc65 skimmer in the first chamber along with a bubble tower that contains mechanical and chemical filtration and i would use chaetomorpha algae in the refugium part of the sump. A mag 3 pump is good enough for a return pump and it is cheap. If you feel uncomfortable drilling a hole in glass, an eshopps nano overflow 300 would do perfectly to get water from your display down to your sump.
40lbs of live rock is a bit excessive unless you plan on breaking some of it up into rubble and putting it in your fuge and first chamber of your sump.
Just hang in there. Learn as you go, work towards your ultimate goals and dont let anyone tell you it cant be done.
maybe a couple of small fish......and some zoas
