I want to run a NS Beta 5 Version with Custom Models and a "Standard" Beta 5 Version on Steam. Is that possible? And if yes, how I do it?

quite simple. install b5, change the directory name, then, install it again. it should, in theory, work, if it dosnt get back to me.

It will work with one more addition

to switch between the 2 you have to rename them back and forth

ex:
to play sans custom models:
NS = normal instal
NScust = custom models

to play with custom models:
NSnorm = normal isntall
NS = Custom models

you could actualy simplify/complicate this and save disk space by doing this:
make a copy of the models dir (or where ever you have cutom things, this might also include sound directories) and then work from there.

ok, i will try that