Can "phase I and II studies" be expressed as "phase I/II studies"?

Yes or 'phase 1 & II studies'.

Nope ...not Phase 1 & II Studies.

Stay with one type of designator.

Phase I/II
Phase 1&2
Phase One/Two

No mixing.

The same as not writing "There were two hundred and 30 people in the tent." or "There were 200 and thirty people in the tent."

Joe(hope it was a big tent)Nation

You are quite right. That 1 was a typo, I intended a I.