i'd be more inclined to get oversteer rather then understeer. At least with oversteer you're front wheels still have some use. (you can always point your way out of the corner and power out), whereas with understeer you've got less options.

As for mixing and matching, if you've got the same on the front two and and a different set on the rear you should be fine.

Don't go putting a D2 at the front left, a tein at the front right, Buddy club rear left and K-sport rear right. That'd be messy.