The 21r was a great all round tyre for a road Seven. I am sure it is unwise to mix them with normal tyres as the car will be unbalanced, swapping them to the rear will be safer for braking but you'll understeer. So a set of four tyres would be good, but what to choose? I do drive in the rain, have 14 inch wheels and have chosen Rain Expert Uniroyals, I am happy ,but it's hard to know if you would be driving my car. As you already have two new tyres, and they are generally well regarded, I think I would buy another two. The car will have less grip than before, but much better balanced than it is now. Alternatively, you can buy a set of four high performance semi slick tyres to get even more grip than the old ones.