Will I experience any problems changing resource %ages on projects drawing from the resource pool, then levelling? What sort of things should I bear in mind, and what common pitfalls are there?

You will not be able to increase % allocation above the maximum level for resources shown in the pool but there should be no problems otherwise with changing percentages.

When levelling the pool however, there may be strange effects on work allocation - always check the hours allocated in Resource Usgae view!