there are so many different types of plecs compatible of living with the royal. It really just depends on what you want-- a "one-of-each" tank where you have many different species, or a colony of all the same type. Either way, the tank won't be very busy except at night. You might want to look into blue led lights for viewing. In a 90g tank, you could definitely keep several cool plecs. don't forget the driftwood!