RE: Stinson Tri-motor
I would expect the .25 equivalent electric motors to run on 3S or 4S at most.
With 6S the motors will spin so fast, they'll practically open a rift in the space-time continuum, unless they're designed to run on 6S.
As far as connecting the receivers, a simply Y harness will do, be cheaper, and add less weight than a SmartFly board. You'll want to disable the BEC on all three ESCs and use a separate battery pack for the receiver, or use a separate UBEC.