Barbie,
I think you will find that pruning barberry bushes is best done with a pair of heavy leather gloves, a chainsaw and a quality spade!
I do not care for barberry bushes at all, if you couldn't tell.
To properly prune anything, you need a set of by-pass pruners. Do not use an "anvil" type of pruners for anything other than dead wood, as they crush the stem.
When you make a pruning cut, cut slightly above a node at an angle.
Barberry bushes take a lot of time to prune properly, as the stems are very small, and there are a lot of them. They can be pruned any time of year.
I don't recall when it is best to prune a butterfly bush, though it sounds like you might not care whether they bloom next season or not, they just need cut back.
You can prune anything any time you want, but if it blooms, you may cut off the flower buds when you do. Some bushes bloom on new growth wood, some on old. I believe that butterfly bushes (at least in my area) are classified more as a herbacious (perennial) plant than as a woody plant, so they probably bloom on new wood. If you get them pruned before they start flushing new growth this spring, you will probably be just fine.
HTH.
(Seriously consider my suggestion on the barberry!)
Dan