I have a few sellers who say that evo 6,7,8,9 have all the same front calipers.

Is it correct? Can I use a evo 7 or 6 caliper with the mounting plate for my 92gsx ?

All evos from 5-9 are the same minor some early differences which dont effect mounting or anything. The X is bigger and uses bigger bolts but can work also,

Maybe this post makes confusion. Why does it state it's for 8-10? Is it for the rotors?
Can I use that plate with evo 6 calipers and evo 8 rotors?

No idea why, all rotors are 320mm from 5-9 and X goes 350mm i think its just easier for them to be a bit more specific but pads are all the same 5 through X are the same up front.

There was hearsay about evo 9 calipers offsetting the caliper a few extra mm but no one has ever mentioned this at all or complained about it

My guess would be that since we didn't get an evo until the 8 in the U.S. they are commonly only listed as 8-9 because 5, 6, and 7 are very rare here and not likely to be found.