Well in that case addressing is the easiest part of all. Just assign some spare pins on the Slave PICs, ground some, and pull others up to V+ and they become the unique address for that PIC. That way, all your Slave PIC software remains the same, and when that PIC powers-up it determines it's own address. Thereafter, the Master just addresses the PICs as if they were standard I2C devices. Nothing could be simpler at that point.