50-60 reb to start, or 50-60 once they've maxed out? If it's the latter, I think that's too low.
Also, don't tie yourself into running slowdown. Some games it makes sense, others it doesn't. Some teams lend themselves more to it, others don't. Perhaps more importantly, I don't really believe that you're thinking re: why you should run slowdown necessarily makes sense strategically, at least in the context of this game.
You say, "I primarily run slowdown to keep those good defenders in", but is there a big dropoff? ... in fact, Golebieowski is a better defender than the guy in front of him. The real question to ask is how your depth compares with that of your opponents', and what the real drop off is on your team. But I certainly wouldn't pre-ordain that you're going to be running slowdown now and in future seasons. That's not optimum strategy.