Memory channel configuration for a r720XD

I currently have a r710 and have purchased a r720XD to replace it. (Wanted more storage and CPU power).  I am wondering if I can reuse my RAM from the r710 and transfer it over the r720XD. The system isn't here yet, But I want to know if I'll need to buy replacement RAM to go with it.

Current RAM installed in the r710 is 18X 8GB DIMS (Part number: H132M).

1) Following these ram guidelines I'm assuming that the way the system works, I will only be able to utilize 16 of the dims, installing them into (A1 A2 A3 A4/B1 B2 B3 B4 / A5 A6 A7 A8 / B5 B6 B7 B8) Giving me 128 GB of ram total. Is this a valid RAM configuration?

2) Is it possible to add the remaining 2 dims in slots A9 and B9 without populating the remaining slots with more of the same RAM? Would I use different slots to put the RAM on different channels, or this simply not a valid configuration?

2B) If just populating 2 of the slots with extra ram is not possible, Can I just buy more the same RAM and populate all the rest of the slots? Would that be a valid configuration?

2x Intel Xeon E5-2680V2 will be installed in the r720XD.

If a moderator wants the service tag of the r710 to check the memory configuration, let me know. It's using the original DIMS.

Thanks for your time.