Actually, it would have to be 65C02 or better. You couldn't do "ldx #$FF" on a 6502, you had to do "lda #$FF" and then "tax" (transfer A to X). The ability to load immediate into the X or Y registers was added on the 65C02.
No, LDX #$FF is a perfectly valid 6502 opcode. 65C02 only added a few minor things like pushing and pulling index registers, and fixed a few minor hardware bugs. It's kind of irrelevant though, since the 7800 has a 6502C, which uses a plain 6502 core.