It is normal to be afraid of sexual diseases to some degree but in clubs like Maxim, Goldentime and Fun-Palast you can feel safer than in small studios for sure.
Of course condoms are available and the girls are generally using them. Like everywhere in Vienna medical checks are mandatory and have to be made on a weekly basis. If however FP and GT management take that seriously or not, one can only guess. I would guess, they do because they got plenty of girls which are replaceable to some degree.
The girls generally speak English, some better, some worse. If you want to have an interesting conversation with the girls you will be better off in brothels like Maxim but you can get lucky to find a girl in the FKKs as well which is willing to spend some time with you and just talk. On a busy night, they won’t.
They are not allowed to charge you money just for talking, though.
If you stay there just to swim and enjoy the “view”, this might piss the girls off. Especially on a slow night. You can expect to be observed as a client in the FKKs. Girls tend to analyze your every move if they have time to do so.