Hey hun, I'd say go crazy ...just remember that if you choose to have figures with faces, have them looking inwards towards your body as this goes with your natural body shape, and makes the tattoo part of you, sometimes when they're facing out from your body, they look uncomfortable, almost like they don't want to be part of you. Balance is good too, personally I like symmetry, but I'm an OC knob lol, I think sleeves look better if they're designed with some sort of balance, having images on your forearm contrasting, matching or opposing those on ye olde Ulna side, thats if theyr'e not designed to wrap around as one.
Generally i'd say there is no real rule to be honest, whatever floats your boat man, it's good to have some sort of theme to go off don't just get loads of random poxy pictures to fill the space...I say get a bit of a plan, then let your tattooist go for it freestyle...he'll love it!!
anyhoo, I'll shut up now...I go way too much sometimes