Optimal multi-GPU system

Dear all,
I am building a 8-GPU system for machine learning (SVM) and looking for guidance/confirmation if I am doing this right.
So far, I am thinking the system would comprise of the following:
Titan Xp (1)
1080 (5)
1030 (2)
One that supports NVLink (thinking SuperMicro)

My questions are:

  1. is it ok to have different types of GPUs, or should they be all the same ?
  2. What is the appropriate way to connect the GPUs ? Would I need SLI or something similar, given that the motherboard and the cards are all Pascal and would support NVLink ?