Mixing Ram?

Yep.

Also more often than not, unless your benchmarking, having more ram trumps using slightly faster ram if you end up with less of it. Especially for Intel processors which care far less about ram speed and in a game like this where many people even report an upgrade from 32Gb to 64Gb is worthwhile if you are using accelerated time.

In the end though we can theory craft all we like, the best bet is try different options and see what performs best in game.