I'm sure it's not a good idea to mix tires. All 4 tires should be the same design/structure.
This is from Tirerack's website;
"As a general rule, tires should not be mixed on any vehicle unless specified as acceptable by the tire or vehicle manufacturer. Drivers should avoid mixing tires with different tread patterns, internal constructions or sizes, and use identical tires on all of their vehicle's wheel positions in order to maintain the best control and stability. Additionally, drivers should never mix winter tires with all-season/summer tires, or mix run-flat tires with non-run-flat tires."
