I would say bridge them for the fronts and up the DB's on the center and you should be rocking. You'll notice a drastically fuller sound than you had with just the onkyo. At least i noticed it with my mono's across the fronts (3 Heresy's). The blocks actually bring a welcomed stage to not only music but HT, so either way you really can't go wrong. With the onkyo preouts and it's internal settings you should be very happy with your purchase. However i will say that it might get a bit tricky with all the triggers running to the mono's from the onkyo.. do you have a clean power block or something along those lines? To me it wasn't a must have but when you come across one at an estate sale for 20 bucks you buy it. It has come in handly with my amping.
With the extra space i think you will more than likely want to initially set it up bridged and then if too much (it could never be too much [Y]) it would be easier to remove then add.
If your fear is that the center will be swallowed don't worry about it, the onkyo will surprise you when you let it breathe.
Let me know if that's helpful!!!