2 GPU plus GT120 in MAc pro 12 core

Can I put 2 GPU (GTX 660ti plus a Quadro 4000) and GT120 as GUI in a mac pro 5.1 12 core For davinci resolve 9.

Will there be any PSU issues?

If not what would be the best GPU for 4K workflow without having to use a Cubix or PCIE expander.

The 660ti needs two 6-pins and the Quadro needs one. You would need an extra PSU. Not necessarily a problem.
Both of those cards are pretty bad for Resolve performance though. Do some more research, and maybe get a 570.

Assuming you're using a full version of Resolve, and not Resolve Lite...

I think you'll get better results using a GTX690 + Quadro 4000 - keeping in mind you'll still need an external PSU.

It would be interesting to do a test - but I think a GTX690 will still out-perform a GTX660ti + Quadro 4000.

Even though it's only used for GUI, putting the GT120 in a x4 slot might also cause performance issues. I've never done any proper tests, but it's worth keeping in mind.

If you're working with 4K timelines though - I think using an expansion chassis is your best bet (unless you're talking about R3D files - in which case a RED Rocket might be a better investment).