There's just so much to choose from!
You could have a pair or trio or comps or calvus and still have room for another breeding pair or small colony of something else. The deal is, comps and calvus take a buttload of time to mature, up to 3-4 years to sexual maturity. And slow to grow. You want to keep them with something that can hold their own against them but won't outgrow them and then own them. They're not too easy to sex either. Males generally get larger faster.
Look into Julidochomis like transcriptus, dwarf marlieri or dickfeldi. Or some of the medium sized Neolamps like N. cylindricus or leleupi. You could also set up for some of the larger shelldwellers like occies, stappersi, speciosus, calliurus or brevis.
Ain't no Tang like a freshwater Tang!