Minimum speed when riding rollers
the source of the bike's stability is the gyroscopic effect of a spinning wheel.

This is incorrect. Gyroscopic forces play virtually no part in bicycle stability. What keeps a bike stable is the counter steering induced by the frame geometry, particularly the sloped headtube and resulting trail in the fork.
In any case, your single speed will be fine on the rollers. It doesn't take much speed to stay stable on them, any gear ratio appropriate for a bike on the road will work on rollers.
I'm surprised you haven't just tried it to see how it would go, and not conclude your setup is fine already.