Live Rock, A little of each or mainly one type

[TRS]

You should be able to fit more than 60 pounds. Most recommendations range from 1 - 2.5 pounds/gallon. Some rock is very porous and others are quite dense. You could order the 70 pounds of Kanai LR and add other types and sizes of cured LR to your FOWLR system later, but that is really your decision.