I would recommend swapping out the 2700k bulb for a 6500k - 6700k bulb because it's closer to natural sunlight. 2700k is way to low of spectrum to be of any help for aquatic plants, the 6500k-6700k is best for aquatic plants.
As for the topsoil, it sounds like you did a good job at washing it, and then using special kitty all natural litter is another good choice for clay. Everything sounds good so far except for the lighting.
15g Fluval Flex - Will be a planted shrimp tank