I dont know if that is true or not. Yes it prevents the breakdown through the metabolic pathway that converts it to DHT. But then it will seek to metabolize into estro.
DHT may have some training and growth advantages that will be missed by using proscar.
But certainly it is best to use it-most importantly for prostate protection.
Proscar won't affect proviron one way or the other. If you're worried about prostate/hairloss then you probably should avoid proviron bro.
As for whether proscar will reduce gains, well probably not but it will reduce hardness and possibly strength and increase the chance of estrogenic sides.