You should not have problems only if you are flying solo against your enemy without your own wingmen.
Inthe Mission Builder, if you compose a wing you need to specify a a/c type from the list. If you pull down th elist, you will only have those default flyables. Add-ons never appear.
When you go to fly your custom mission so created, you will end up say, you being Spitfire MkXIV whereas your wingmen are Corsair! Try and you will see what I mean.
I guess, there is a way to modify the .AIR file of the imported a/c so that they will appear on the list. So you can have a wing of 4 imported Spitfire Mk XIV for instance. Get it?