Your work is some of the best on this forum! I love the whole monsters tickling girls motif. May I make a suggestion? Have a girl be tickled by a whole room filled with dusty skeletons. Either that or a mass of slimy tentacles (don't have the girl restrained by chains or anything. Have the tentacles hoist her into the air and tickle her all over). Or maybe just a big scary monster, like the one portrayed on the Coney Island haunted house (although the one that you use should ACTUALLY be scary). Or anything from the Dungeons and Dragons monster bestiary or Book of Vile Darkness (no I don't play D&D, but I have friends that do and I saw the books and they are cool, especially the Book of Vile Darkness. I SHOULD play, but I never really had the drive to figure out the rules of the game).
Another thing you can do is make the girls where normal everyday clothes, and then have a series of pictures where the monster(s) strip her while they are tickling her. Having that gives opportunities for shoe/sock removal, and you can have a monster slip its hand or tentacle up a girl's miniskirt and tickle her panties, even sending the tickling appendage inside her panties. As for what clothes you can have the girls wear, normal everyday clothes are fine (T-shirt, jeans, sneakers, and any combination that you would see a girl on the street wearing). I, personally, am also biased towards cheerleaders and girls in formal prom gowns (18 year old and over, of course), and against high heels, if anyone cares about MY preferences, but whatever the case. I also think that making the girls look 18-22 would also be great, but once again that's MY preferences (then again, what do I care about anyone else's).
Anyhoo, great great great great job!