If you have to mix and match, use 5xxx/61xx-67xx and 68xx-69xx OR 79xx. That way you can match sdk's to their optimum card. 2.1 works best with 5xxx/61xx-67xx, 2.4/2.5 works best with 58xx-69xx (2.1 is incompatible), and 2.6 sucks but is required to make 79xx work. You can also install 2.1 plus 2.4 OR 2.5 OR 2.6. You can't have any combo of 2.4, 2.5, and 2.6 installed in the same system because they install to the same path.
Do NOT combine 79xx cards with 68xx-69xx cards; you will lose mhash on your 68xx-69xx cards because you'll be requires to use 2.6 to use 79xx, which also is much slower than 2.4/2.5 for your 68xx-69xx.