I was last in Frankfurt in July, you could try looking at http://internationalsexguide.info/forum/
Fkk's it depends what type of girls you like, Sharks has a good mixture, decent facilities and not so sharky girls even with the name of the club, Palace is sharky, with girls who constantly will try to upsell, but if you know how to deal with them it's not so bad, they do have a high proportion of fake boobs, Oase can be good to, more natural look of girls, good facilities and big variety of girls.
For laufhaus scene yes you just wander around the corridors of the buildings until you find someone you want to punt with, no obligation to go with any, the girls will usually be sat on a stool by the door looking sexy or in their room, if the doors shut it means she has a punter already.
There are several German forums for punting, Hurentest
is the biggest, if you don't know German use google translate, take a look at my map of Hessen https://www.google.com/maps/d/edit?mid=zu1tlMSEJdqg.kk3xtIn4d_LY&usp=sharing