If you go for shellies, the best for a 10 gallon is n. multifasciatus because they are the smallest and relatively peaceful. You'd probably want to go for a single male with 3 females. I wouldn't personaly add anything else to this sort of set-up and I personaly wouldn't have it planted either but, otherwise, a small schooling fish would be fine but watch you don't over-stock.
If you want something unusual, an african butterfly fish would work. As it gets to 4", it'll fill out the tank a bit. You could put in a couple of kuhlie loaches to fill in the bottom as well. Anything very small will get eaten by the butterfly.
Killies are also quite uncommon and there are many that would do very well in a planted tank and that you could breed. Research whichever you are interested in thoroughly first.
There's a whole host of gouramies that could work and some can be somewhat aggressive. A single three-spot male is an example. Don't keep anything else with him if you do this as it's quite a squeeze for a 6" fish and one that is likely to kill any tankmates it dislikes.
Whiteskirts are a schooling fish so you need 6 at least and that would mean your tank's already over-stocked. No shark will fit in there either.
Hmm... are you interested in dwarf puffers? You could keep 2 in there and maybe get some freshwater bumblebee gobies to go with them (TBH, you should just keep the 2 puffers and nothing else though).
