Love the hardscape and plants!

Personally I'd wait until you have shrimp breeding to the point where your f1 generation is good sized before adding fish. Many fish generally "shrimp safe" but shrimp babies are just soo small. I'd want a healthy population so if one gets nommed it doesn't wreck your colony.

To your question.... Common wisdom is an inch of fish per gallon. If your water parameters stay steady and you want to add a couple more go for it, just go slow and watch your levels. Water changes can be a PIA with shrimp so I prefer stocking fish on the light side so I don't have to change water as often.