Hi Kim,
Your tank size/stock is OK. Sand is better than small gravel, if only small gravel make sure is is well rounded (i.e. not sharp). This goes for the Corys too. The best option I have found is lots of bogwood with plenty of hiding holes.
This is what I did.... Light substrate with nutrients for the plants, then rounded gravel areas and fine sand areas (the joins are blended). With plenty of bog wood, plants, and a few rocks you can end up with a really pleasing look. Corys go over everything (gravel, sand, plants, rocks), and the Kuhlis spend 90% of there time in the bogwood, and the rest out and about (mainly in the sandy areas next to the bogwood).
General water conditions are: pH 6.5-7.2, Temp. 78-82F, Hardness 12DH. However pick the best you can find from stock that has spent time in water conditions similar to yours. My LFS has pH 8, 78F, 20DH, so my Khulis are very happy in pH 7.8, 77F, 15DH. This is not a big deal, just good practice.
One note, Kuhlis will not be on show all the time like your other fish. They will just pop out now and again to remind you that they are alive and how special a creature they are
If you want something else that is on show and lives mainly in the bottom 3rd of the tank Cherry Barbs could be a very good option.
Best.
George
