You can divide now if you want. I would always add compost to any new plantings. Yes you may affect the blooming for this year. Depending on the size of the division you may get no flowers, a few or almost as good as last year. I am sure you would not be dividing without a good reasom. Al

The reason I want to divide is because they have gotten too big where they are.

Unless you want to relocate the plant you can just cut away part of the plant in the ground without all the work involved with digging up an entire well established peony. Al