You could do that, yes.
As to the point about turning combat ships into multirole ships - I don't think it makes them better multirolers; no moreso than multirole ships can already handle combat. It's an awkward delineation that makes no sense to begin with. A ship is a ship. If anything, multirole ships already prove this point perfectly.
Either have combat ships, cargo ships, exploration ships, etc, without any multirolers or just have ships that we outfit for different things. As it stands, the multirole ships just do everything as well or better than all the specialized ships. Which is why multirole ships dominate the ownership charts according to INARA.
Or just remove the concept of 'roles' altogether and all ships are just, you know, ships.
What makes them suited for a role is their general layout. The current 'multiroles' only exist because we have specialized craft to start via locked slots. Nixing locked slots and using modules to determine roles makes more sense, given Elite ships are designed with a 'plug and play' attitude anyways.
Balancing a ship then becomes less about roles and more about its core attributes, which should have mattered in the first place: armor, mass, agility, shield, etc. Instead of trying to balance something as convoluted as slot quantity and size, reduce the formula down to just core modules and their base stats. Then, leave it to min-maxxers to find "the best" ship for whatever (as they did with Python, for example) and the rest of us to just build the ship we like.
I really want to explore in my Chieftain. I love the look for it and its lighting. Even with unlocked slots, it's range would be decent but far from great. Most of your multi-roles would still excel because the majority of unlocked slots are still size 5 or smaller. Most of what makes multi-roles good at what they do is the fact they have multiple size 4 or larger slots.