welcome to this forum. Nice to meet you! In which part of the world do you live?
: So as far as i see street skating on rollerskates is possoble too, right? Wich tricks is possoble grind on curbs and rails?
You can do quite some stuff on curbs on rails. But i would say that hang-ups are more likely on roller skates than on a skateboard or on inlines. You also can't disconnect from your roller-skates so that you have to be prepared for some nasty slams.
You can see an example of what can be done in on of
Bombe's Lyon videos.
: And about the skates it self.
: As far as i see i can just order a frames and put it on any aggressive inline boot, right?
You can use inline boot or hockey boots or basketball boots or dedicated roller skate boots (I believe there are some nice boots from Riedell).
: So i found some frames in stores, look:
: First one has a longer base and looks cooler. But i cant see the space between the tricks, mybe its too deep to slide on rails. What can u say about all this?
You could start out with either of them. The first one would be suitable for sliding but as you noted it is quite deep between the trucks so you might want to add a riser. A piece of hard plastic would be nice. You could also make a groove into a block of plastic which you can use as a guiding aid for sliding.
The second one is suitable for grinding on the axles as well as on the grind bar. For sliding you might want to put some plastic on it. Or maybe you prefer to grind in both directions.
You can also find some hints on building your own skates like mine or Tim Altic's. Toby has collected quite some info on trucks.