I'm not personally experienced enough with shellies to give you an accurate number for a 27 gallon ( BTW- bowfront? ), I'm certain someone else will be able to help you there. I've read recommendations for 10 in a 40 gallon breeder.
As for Brichardi, they are on my list of fish for a dedicated species tank, they are so prolific that your tank will quickly become overrun with them. Mine have been moved out of the community 55 into their own breeding tank, which is now chock full of the horny little fish.
For tank mates, in your tank I would look into Rainbows.They are topwater fish , and would give you the balance you're looking for... they're not Tanganyikan, but honestly, at 27 gallons there aren't any I could recommend as tankmates for a shelldweller colony. Most grow too large for a 27, outside of keeping a single pair. I do think you could possibly get away with 1 Julidichromis Transcriptus, though, but I would think that would be fully stocked with the Multis.