let say i can land cat2, but minimum weather must be good for cat1 (200ft/550RVR).
Yes, but not all CAT 1 ILS approaches have RVR minima as low as 550m, you have to look at the actual relevant approach minima. Also, from a legal standpoint, the cloud ceiling doesn't matter when talking about precision approaches.
if i can land only Cat1, i will use minimum weather for a VOR approach.
Yes. Assuming the airport in question has got one; or use the relevant NDB/SRA/LLZ only approach minima as applicable.
in this case i can file the airport as alternate?
but I can still land the way I want?this is only for planning?.
Correct. Once airborne, you need only worry about the actual weather/visibility to determine the legality of an approach at your alternate (or continuing to your destination, if your alternate airport no longer has a usuable approach).