Here's the basic answers
The fly rod casts the fly line and fights the fish. So you need to pick a fly rod action that suits your casting style and will still be able to tame the species you are after. A rod has to do both jobs and not just one. For example, you would not pick a slow action rod to fight a tuna, even if you like casting slow action rods.
You choose a fly line based on the taper of the fly line AND whether it sinks or floats, or does both as in a sink tip fly line.
The choice of the taper depends on what fly you are casting AND how gently you want that fly to land. For casting heavy and/or air resistant flies as you may do in bass fishing,you want a "bass bug taper" line with taper designed so the line mass is toward the front of the fly line. For spooky trout in clear water, you want a very gentle and long taper so the line lands with minimal disturbance.
Specialty taper fly lines are more expensive than a generic Weight Forward (WF) or Double Taper (DT). So I don't think you are going to find a Bass Bug Taper fly line that is very cheap. You might find one on sale but it will be more expensive than the generic DT/WF fly lines.
So the short answer is that if you buy a WF/DT fly line that works for trout, you will limit your ability to cast the heavier flies.
One of the best if not the best line for casting heavy flies is the Rio Clouser.
Cabela's - RIOŽ Coldwater Clouser
Here is the taper profile of the Rio Clouser and you can see the taper places more mass forward than the generic DT and WF tapers below