I'll shed some light on this. I was in the same boat.
I've used S5 for 1.5 - 2 years. I've lost some ground but it has helped me hold onto a lot of my hair (or maybe it's the minoxidil and S5 does nothing) It's hard to tell when you're on multiple things.
As for finasteride, I started recently this year and immediately felt libido sides. S5 and finasteride are in different ballparks. In my anecdotal opinion, you should be fine on S5, throw it on there once a day after minoxidil, you will probably feel no side effects. It's topical and doesn't enter the bloodstream or if it does, it's very minuscule amounts that will not affect you.