Shamrock, for panniers it's good to have three struts so the panniers are less likely to wiggle.
I had one of these and liked it, the lower rail enables you to attach and remove things to the rack w/of interfering w the panniers
http://www.topeak.com/products/Racks...tDXTubularRack
I haven't used one of these but I found for my CrossCheck that it handles a load better with small panniers close together on a rack like this
http://www.blackburndesign.com/racks...l#.UxKIRX-9KSM
I'm using a discontinued Axiom rack like the above one but using ss mounting straps instead of their clunky hardware to the seat stays .
btw what kind of bike do you have? Mismatched tires are no big deal.