I would recommend that your never mix tyres.
Although I have very limited knowledge compared to the true tire-philes on this board, I would 100% agree that mixing tires is a dangerous game.
I just put a set of Bridgestone S-03's on the 355 and really like them. Significantly more grip than the Expedia's. Put about 100 miles on them and then went to the track. For a street tire, they did very well on the track...they aren't Hoosiers, but they handled well, were grippy and were very predictable. I've used Toyo's, Kuhmos, Hoosiers, and Bridgestones on my race car (albeit lightweight race cars - stripped '72 911 w/ 3.2L at 2100#) and if you trailer your car, Hoosiers are the way to go. But I agree that it depends a lot on personal preference and what you are after (max performance vs cost vs treadlife, etc.).
