Building biceps

I have always had pretty big arms for a girl, but decided to start working on them when I joined the gym in November. I do some drag lifts for my shoulders, and use the dip and tricep machines for my tris, but use free weights for my biceps. I am pleased with the progress so far, after a workout they are now all swollen and squishy. My forearms are starting to look like Popeye, which is odd, but I suppose they are useful for stabilising. The problem is that all (or at least most) of the improvement seems to be on the inside head of the bicep. I do hammer curls and bicep curls, 4 sets of 10 each, and then train to failure if I still have some left. I don't know what weight the dumbells are, but they are number 7 out of the set of 10, so not super heavy.

Anyway, I am after some exercises to balance up the inner and outer heads of the bicep. I have recently started a close grip on the barbell, although I find it really uncomfortable on my wrists. I can't do any lift where I raise the weight to chin height atm as I have some rotator cuff instability that I am doing exercises to correct. I am still very wary of using any of the benches and stuff in the free weights area as I find it intimidating, so if the exercises could involve just a couple of free weights that would be cool

Any advice would be appreciated.