50/17 is big but not extreme. If that's what you like SS, you may wanna keep it for fixed as well, though trying a smaller one surely can't hurt.

Most people seem to get into fixed instantly, or never, but I'm one of the few who only got to really like it after a good 2 or 3 weeks of trying... In that time, I was sorta undecided.

When I got my trackstanding, bunnyhopping and general bike control at all sorts of speeds properly down, I started really enjoying fixed. I don't miss coasting much at all (urban riding, not centuries and tours).

Knee problems: Not an issue unless you're silly. Leave your front brake on and use it for emergency stops; no need to skid all the time. Don't push a too big gear ratio, don't go climbing hills fixed or ss with your usual ratio. Just not coasting won't kill your knees.