Have a 5 unit rental w/5 boilers. 3 @ 105,000 BTU & 2 @ 79,000BTU. Need to vent all units on 1 power venter.?

Thought field control SWG8 or Tuenlund HS3 would suffice. Problem is when only one 79,000 BTU unit is operating, HS3 or SWG8 will cause too much draft - what are the options other than using 2 smaller power vents. (chimney unuseable)

Is it possibe to reduce power to power vent blower motor on a rheostat or similar?You would have to build a sensing circuit to increase fan speed as demand increased.