With a tank that size there is nothing to stop you keeping clowns, especially at the size you will buy them at and they are medium speed growing so odds are you will have a bigger tank by the time they need a bit more space. I had a standard 3' community aquarium with 8 clown loach in it in addition to a shoal of about 15 emperor tetra, 6 corydoras adolfoi, 3 golden gourami, 8 marbled hatchets, and a mixture of other barbs and tetras - probably close to 80 fish all up. It was running with two fluval 403's at the time with plenty of aeration and water changes - water quality and parameters were always pristine and never had any losses. Its a lot about choosing compatible fish and doing proper maintenance when you run a community tank - the emphasis on the word "community". Good luck, and I hope we get to see some pics when its at a stage you are happy with.
Incidentally, its a tall tank ! You could opt to have the water level lower, say at 400 and have plants that grow up and out of the water, or do it along the lines of a swamp/mangrove biotope and put things like archer fish in there as a feature (assuming you can get them here) - have a proper lid or at least covered with mesh and you can put flies in there etc, which the fish will shoot jets of water at to catch them for food - way cool IMHO hehe.