I have read that mixing slow and fast miners on a vardiff pool is a bad thing?

I have one rig mining Max for the past day or so, thinking of swapping my other rig over for a day or so also.

It has 2-280x and one old 5770 (pulls about 200kh on scrypt)

Now if I was to swap to a vardiff pool would the slow miner break the retargeting for my worker?

If so, is there a way in cgwatcher to tell just that card to submit shares to another worker?

Any info would be great. Thanks