Well do whatever weight and sets you can nail down good form. The low cable will help you isolate just the "rowing" portion of things so that should help you later on convert that into better bb rowing form. If you really feel unnatural with it then use moderate weight lowish reps so you can work on quality sans fatigue from higher reps. Then when you feel more confident convert that into higher reps...even up to 15 if you want.
I think a lot of people do bad cable rows out of ignorance and so they cheat and whatnot. But it's easier to correct these problems versus not feeling so good with a barbell because you simply have more to deal with. So to me it's sort of like when you learn olympic lifts and they have you practice each portion of it....just not near as complicated of course.
I think the goal should be to try to get all your rows heavier somewhat but mainly to be comfortable with whatever row you're doing. And if you feel "unnatural" I'll bet 9 times out of 10 that translates to "need lighter weight".
Don't be afraid to do more than one kind of row in a session. I.E. you can do some practice work with a row you're learning with moderate volume and then finish up with a row you more comfortabe with.
Hell the other day I did cable rows w/parallel handle followed by V-handle followed by barbell....(followed by keiso shrugs
).