15g is the size of a big bucket so not large. I don't know of any fish that eat dead or drying (I assume that's a typo and you mean dying?) plant matter. If you have dead or dying plant matter, remove it by hand, or net it out. If it is drying, you need more water
Kuhlis love to dig. They will dig into whichever of your substrates is smoothest and easiest to dig.
Siamese algae eaters and Ancistrus (bristlenose) will get too big.
There are dwarf corys out there, 3 panda corys would be a suitable size but are more expensive than the larger ones, but I am confused by your comment " they are bottom dwellers like the existing tetras". Neons and serpaes are not bottom dwellers. As you were considering algae eaters, ancistrus and corys I thought you were looking for bottom feeders.
Tiger barbs will be very aggressive in such a small tank.
No, you can't get freshwater shrimps here (not tropical ones anyway)
Kuhlis and panda corys are the only bottom dwellers I can think of suitable for a small aquarium.