If you want good separation between your quadriceps and hamstrings, first make sure you are concentrating and that you’re really feeling weight on each rep. That is how you will create more detail. If you lift with explosive contractions, you won’t really fell the muscle, just the weight. You will get bigger but not necessarily better separation. For legs, use high repetitions on drop sets to fully engage and fatigue the muscles. Aim to do more than 15 reps per set so that you feel the burn. Plus, high rep training for legs helps to burn through more glycogen (stored sugars in the muscles) and total calories to help you get leaner overall.
I think it is a good idea to add one of those mass builders in there. For one, with the romanian deadlift the hamstrings get worked from the hips, not the knees. This means you are working them completely in a different manner than with leg curl movements, even though both target the hamstrings. It helps with shaping the hamstrings and your glute-ham tie-in as well. I don’t think RDLs add too much mass to the bodypart it gives the illusion, which is important in bodybuilding. Sometimes the tape measure does not matter. It is all about the appearance.