First off welcome to the cafe.
Its osunds like you are loking at MR's, and yes they would make an excellent sub for those days when your SP's don't throw. Its actually a very common strategy for Yahoo, and very much recommended.
As for player subs on the days when your regulars don't start, try and get a guy with multi position elig. Ideally a guy with a good BA and batting near the top of the batting order. since he is a sub, you cannot rely on the power bieng there whe you sub him in. A good BA and a few extra runs a week is always beneficial.
again welcome aboard.