As it happens, I'm about three episodes away from finishing ReGenesis. I watched it in inside out order (third season, then fourth season, then first season, and then second) which is not a bad way to do it especially since it goes a wee bit completely bonkers at the very very end.
Off the top of my head -- besides Mayko as a regular -- Patrick Garrow (Turin) is in a couple of episodes in season 3, Rob Stewart is in a couple of episodes in season 4, Weej can be spotted in the background in a season 4 episode, I'm pretty sure I saw Sean Baek in there somewhere, but I don't remember which season.
And going the other way, the dude that plays Bob in ReGenesis is almost unrecognizable as Coren the heavily bearded Space Pirate from Killjoys season 1 who was Dutch and Johnny's pre-credit's warrant right at the beginning of the first episode and then showed up at least one other time.