I'm currently setting up a 20g long tank and wanted some advice on stocking.
It's not going to be a planted tank and will only have plastic plants. The substrate will be mostly black sand and some gravel mixed in. I have an aquaclear power filter rated for 5-20g, aqueon 50 watt heater rated for 20g, and a bubble maker. I'm actually having a problem with the filter right now, it keep sucking up the sand and clogging the motor. Any recommendations for a different filter which will prevent this?
Since I want to have glofish I need an actinic light, but I also will have other fish so are there any lights which have multiple or duel operating modes? I'm assuming that regular fish aren't going to be happy with just actinic lights.

Stocking:

10glo-fish
4-6 cory catfish - is there a specific species that stays smallest? the pet store I went to had several different cory varieties.
if it's at all possible I'd like to fit a rainbow or swordtail shark in, let me know if they will be too big for the tank or too aggressive.
If the sharks are no good, can anyone suggest another fish, darker colored fish to contrast with the glofish, that will share the water column with them.