I got things figured out and now have 4 PIs running a totall of 10 U1/U2s and 1 U3 and still have one pi idling as a standby. THe U1/U2s are running under BFGMiner and the U3 is running under cCGMiner.  I working out a stability issue with the U3 where it will run for several hours and then the process will stop with no apparent error. Not sure if it's the PI, CGMiner, or comms...