Yeah, I did the math. You want engines at least 10x the mass of the tractor, preferably 20.
Below a certain size, you want hangars to move ships with zero or boosted engines, and above a certain other size, you want tractors.
Something I had been thinking about:
Tugs that are not towing a ship or base are very fast. It is possible that if a ship attacked a tug-freight pod combo, the tug would be able to get away, where a freighter never would. On the other hand, if there are a lot of completely stationary civilian ships around, boarding pods can be much more effective against them.
And I hesitate to consider the morale implications of serving on an engine-less freighter, knowing the tug will ditch them like a salamander's tail at the slightest sign of trouble!