group only if more than 2 windows

If not already possible, can this be added:

group windows only if there are more than 2 (or N, configurable) in a group

For example, if there are 2 windows of some app, I would rather have them independent on the taskbar, but if I start 5 instances, it would be better to have them grouped.

PS: Explaining the difference between grouping and combining would be nice too ;-)