Digital RGB interface EMI filtering
Answer 1:
If you don't know anything about the impedances I would always use series termination in the range between 50 and 100 ohms. Which value you choose is not that critical, but as a rule of thumb: When the communication still works with a higher value, choose that. EMI is caused by reflection due to impedance mismatching. The intended Signal will have to flow through the series terminator only once, whereas a reflection will have to do this at least twice causing it to shrink. So series termination is no reflection prevention but an effective and easy way of reducing it.

Don't try to attach an oscilloscope to evaluate signal integrity, since this will significantly influence the signals.

Answer 2:
The frequencies you mentioned can be suppressed using ferrite beads. If space is no constraint, try the one you can wrap around FCC cables.

Power lines should be buffered with capacitors on both boards, make sure to use caps that are fast enough. It is common practice to parallel multiple caps (say 3x100nF) to reduce ESR.

If you still have problems with the clock you can try to add a low-pass at about the clock frequency before feeding it to the cable and restore/buffer the clock at the sink board if necessary.