I am helping install a new Comfort-Aire GeoMax 2, 3 ton, open discharge system.  This is new for the heating contractor too, so I am being a guinea pig and know it.  His supplier sent two Taco valves which is fine, but also 2 hays 4gpm water controls.  Don't I need a 4.5 for the first stage and a 1.5 for the second stage making 6 total?  Why can't I just use ball valves, which could actually be adjusted to fine tune the system once it is operating?  Thanks in advance, I'm sure I'll have more questions.   

Globe valves are adjustable, or gate valves are, but ball valves are very difficult to adjust to flow. Not that they absolutely can't be, but it's difficult.