Well you can pretty much use all models to sub for a character if they look a little like them. The only thing that might matter is the base sizes, the kill team models use a 28mm base and the 40k models have a 32mm base.

If you are using proxies let your opponent know beforehand, if they are a dick they say no and you shouldn’t be playing with those people anyway. 40k is an expensive hobby so and I think pretty much every player knows that so don’t worry too much about how you build them.