Ok, I can close this post, we're replacing both 3.5" 6100XVS with 2.5" 6100XS units. All I can say is wow. just wow. Something changed identifying the chassis type in the 7.x firmware and it put the nail in the coffin.
Hi, I have two PS6100XVS and two PS6500 (one with 3TB and one with 2TB drives) The 6100XVS has 17 600x15k and 7 400xSSD drives.  One Pool is setup with a 6100XVS and a 6500 2TB Drives, this pool HAD 6x firmware and was then upgraded to 7.1.2 It is currently working correctly as a hybrid drive/sumo system. The other 6500 with 3TB drives was upgraded to 7.1.2 from 6.x firmware and then a couple weeks later we purchased a 6100XVS and upgraded the firmware to match 7.1.2 We are unable to add the 6100XVS to the 6500 pool - it just hangs on "initializing" No matter which way we approach the system it will not add in to the system. In testing, we've identified that if we downgraded everything to 6.x firmware we can introduce the 6100XVS into the 3TB6500. HOWEVER, that system is a remote production system and we are unable to take it down and to the crazy dance needed to evacuate/ downgrade/evacuate/introduce/upgrade. THEN the question is what happens to my existing 6100XVS that was upgraded while in operation? What happens if a drive fails, or if we need to re-purpose that system? Having said all that when we tried the same operation with a 2.5" 6100XS everything went swimmingly, took the drives out of the 6100XS and installed to the 3.5" 6100 and was able to make the system introduce... so it appears that there's something in the 7.1.2 firmware that prevents the 6100 from using the 15kRPM drives in a hybrid system. PLEASE HELP! I love the Equallogic gear, but this is wreaking havoc with my symmetrical dreams over here!
Updated 6100XVS to 7.1.2 now it won't initialize or join a pool