Will these fit OK with a Sierra Crew? & Flow Rift Boots? Im a size 8.5 looking to get a medium

A large fits 8-11. Depending on the volume of the boot, it would be safer to go with a Large and size the heel cup short.

Yeah man you should be good to go. Ride makes their bidings fit the best with Ride boots(like most companies), but obv they are compatable to almost every boot out. You will just have a lil more play in the cage of the binding(foot bed, heel cup and sides), but not overly much. I had a pair of 32 Lashes with a pair of Ride Beta's and there was a lil more room than I would have liked. But to be honest it ddnt make any difference to me because I have size 12 boots and L/XL bindings. So smashing my big ass feet into larger bindings was ok for me. But these should be just fine for ya. Hope this helped bro