Is it too much to have floor standing towers for the front and rear?

No, not at all. It won't be any louder than using bookshelf speakers. At least not perceptibly anyway. You should be able to adjust the level of each speaker with your receiver's setup menu anyway. Theoretically using larger speakers is advantageous as presumably they will have a better frequency response and should, in general, have better fidelity. I'd use the JBLs on the front. They are probably more efficient and better quality than the Sonys.