Honeycomb catfish are ADORABLE, but are the most timid catfish I have ever seen. I have one in my 20 gallon long that I have tried in a variety of settings with lots of peaceful tankmates and have never managed to coax her out of her hiding spot other than once in a blue moon when she's not able to snag enough food in the shadows. I honestly thought she was dead for about 6 months until I rediscovered her during a tank re-arrange. I really don't recommend them, which is a shame because they're so funny to watch swim on the rare occasions mine comes out.
Substrate-wise, I'd recommend sand due to the MTS, shrimp, and Kuhli loaches if you go with them. MTS like to burrow around through the substrate, as do loaches, while the shrimp have an easier time keeping the bottom clean since the food won't sink through and out of their reach like it would if you used gravel.
Another potential tankmate you could think about would be the Mexican Dwarf Crayfish. I just found out about them recently when planning my shrimp tank, and they're quite fun. They stay small, won't bother anyone else, and come in an eye catching bright orange. People keep them with RCS and the shrimp often will ride around on the crayfish's back
Edit: Almost forgot! Check out "Fissidens Fontanus" for a nice short green plant for covering the caves. It's MUCH prettier than Java Moss as it has a sort of "fern" pattern to it's leaves. You should be able to find it on Aquabid.com pretty easily. Stick to the US sellers for the best results
