If he signs, Schroder is one of the C's 4 best players so he should start if he is signed.
You'd almost certainly start Schroder, Smart, Brown, Tatum, and Horford in that scenario with Richardson, Nesmith, and R. Williams being the core 3 off the bench (match-ups would dictate which other guys you played more). And even if you wanted Schroder on the bench to start, he'd be playing starter minutes because he is one of the 4 best players on the team.