Firstly, the fish you buy in the shops are really only babies and will grow a lot bigger over time and fill the tank more. Also if you have both male and female mollies and platys you may get babies to add to what you have. Angels and dwarf gouramis are soft water fish and the fish you already have prefer hard water, so really shouldn't be mixed in the same tank. Also, angels are likely to nip the trailing fins on dwarf gourami so these two wouldn't usually be combined. Small angels will grow into big angels also, and if they happen to be two males you may have problems with them fighting. For that reason, it's usual to keep just one, or a group of six so you get less aggression to any one fish.