If you are primarily going to do stems, then gravel to anchor them would be fine, but you'll still have to fertilize.
If you want to actually plant them, then I'd do a dirt layer capped with either fine gravel or sand - but stems get nutrients from the water column, so dirt is really a waste for them.
For a dwarf lily or lotus, you'll have to use a small container with soil the consistency of clay and no potting soil. I'm not sure either one of those will work in a container that small, but you could try and then you'd know for sure. That might need to be the only plant you have in the pot.
The highest water temp for fish would top out around 82-84 degrees, and any fish would need some plants to hide under to avoid cooking in the sun.
I've ordered pond and bog plants from dragonflyaquatics.com, and been pleased with them. I don't know if they have the lilies or lotus you're looking for...