I got some of those little cleaner bug things at LLL reptile. I don't know that it really did much good, but i also haven't done a fully "natural" set up. I just spot clean. If you don't just spot clean you are gonna have one stinky mess on your hands as it breaks down. I think i also remember you said you were doing mulch or like compost in there, too. I'd be more concerned with that smell than the size of the tank.
75 gallons is fine. Why not? He's going to have lots of things to slither around and have the snake explore. I know i have two snakes who sit there and don't move, and two bps that love to roam around and get excited if i move sticks or things after cleaning. The one is in a 60gal and he loves to roam around sometimes, and climb.
to back up....if you had to slither on the ground i don't think you'd like to smell compost or old feces, or mulch or whatever *lol* if you've been doing this already, does your snake climb sticks or sit in places that don't stink? maybe it doesn't bother the snake, but compost and decomposition do set off gasses...probably not a lot for just the bottom of a tank, but i think your snake would have less health risk without all that.