TC - I don't think there will be any problem with lighter bullets, except they might be more difficult to find an accurate load with - hard to say. The twist is definitely geared toward heavier bullets, though.

Consider the smaller diameter of the 20 bore's rim in terms of square inches compared to a 10 bore's rim. That's where the difference in action strength comes. A 20 on the 10 bore action will handle more than the typical 11,500PSI as a regular diet. By the same token, if the 10 bore was re-barreled to something like a .22 Hornet with it's tiny rim, factory ammo could probably be fired quite safely - at 44,000PSI.(of course, the firing pin wouldn't work as it's too large and would blow primers, etc. - the point is the action would take it as the force AT the ctg. head is the deciding factor. Now, the straighter walled case results in the brass being able to adhere momentarily to the chamber walls at peak pressure which reduces bolt or action pressure, ie: bolt thrust, action thrust. The barrel must be up to the task of handling the pressure which is relieved from the action, which it usually does, in stride.
This is just one of the reasons Ed H. is getting such fantastic results with his 3 1/2" brass shotgun hulls. Minimal taper, stronger cases, stronger primers, etc.