Allan, that is a good list!
I ran into one more to add to it:
The new DIMMs are not compatible with the old DIMMs.
As a check against this and some of the other items on Allan's list, I suggest installing only the new DIMMs, and see if they all are working. Then you are starting with known-good modules. Then you can add back the old ones and see if everything still works.
For 2008 and previous, they go matched pairs in adjacent slots on the SAME Riser, Densest usually go in the first slots.