Testosteron gets converted by enzyme 5-alpha reductase into DHT which is harsh on hair. Testosteron itself is also harsh on hair, but DHT is much harsher.
Nandrolone itself is about as harsh on hair as Testosteron, but gets converted by same enzyme into DHN, much, much milder on hair metabolite.
Finasteride(Proscar, Propecia) is 5-alpha reductase blocker, it prevent convertion of Test to DHT (which is good) and convertion of Deca to DHN (which is very bad)
So, using Deca with Proscar is not recomended, you will get about same effect as using Testosteron without Proscar=hairloss.
I would suggest not to use Deca. Dropping Proscar for the cycle and a few weeks after will work, but it's not recomended, you know you have to stay on Proscar, also, you have Test in your cycle, so even if you drop Proscar, you will have nothing to prevent hairloss from DHT which is even worse.
Drop Deca.