Sorry Emma, just to be clear are you asking if you could use normal polishes with your gelcolour topcoat?
If that's the question then yes, perhaps you could but you wouldn't really be advised to perform it as a professional service as you are not following manufacturer's advice on how to perform gelcolour service, mixing systems and not using the full gelcolour system. You couldn't charge the same or expect the service to last as long. Plus you wouldn't be able to topcoat immediately as polish takes a while to dry.
That said I have Shellac'd my nails before including topcoat, then put CND 24k gold polish over it (like a glitter coat) then waited 24hrs before putting another shellac topcoat on it, and it worked really well. I only did this on myself over Christmas, and even though it worked wouldn't offer it to clients as it isn't the proper Shellac service.
Apologies if I've totally misunderstood your question! Xx