I would say that Scouts overrides Sewer Rats - they'd start on the board, in your deployment zone, and get their free move.
If I was playing you, I'd allow you to hold them in reserve with Sewer Rats, and then deploy them from a sewer grate - but you'd forfeit the scout move (Scout Move occurs before the first turn - if they're not on the board, they don't get the move!).
Don't have my book - is there anything in the Scout
USR that says "may"? Are scouts *always* deployed on the board regardless of reserves etc? Or is it that they always have the *option* of deploying on the board, regardless of reserves etc?
I'm pretty sure that Sewer Rats is a 'may' - you can hold some of your units back, but you don't have to hold all of them back.
No way are they getting a free move midgame when they emerge from the sewers! Nice try...