I would keep the platies, dwarf cories and dwarf gourami in the smaller tank. You would still be slightly over-stocked but it should be manageable with regular maintainance and water changes.
The bristlenose is a good cleaner for a small tank but not for one that is majorly over-stocked. I'd move it to the larger tank with the cichlids. You can keep those barbs as well if you want and just increase the shoal size for the larger tank. If you decide to return them after all, you may consider some cherry barbs instead. The cichlids will likely reach around 3 inches in length so they should be ok in the larger tank with the pleco and 5 tiger OR cherry barbs.
Another option (though many will argue) is to keep the dwarf cichlids in the smaller tank and move all the other fish to the larger tank. Then take out the barbs and add some more dwarf cories instead as they are realy shoaling fish. This may cause problems with aggression between the cichlids due to limited territory/no distractions (other fish) but it might also be a better option as I think you wouldn't be overstocking the smaller tank and could give those cories some friends...
If it's at all possible, you should try to get a 20 gallon rather than 15. The difference in price can hardly be compared to the huge benefits the larger volume provides. It also looks more impressive and allows for a wider selection of fish in future.
