No reason at all not to plant under the trees other than leaving a path to the cherries. The species peonies like some shade and do well under the cherry. The pear tree may cast heavier shade and may be more suitable for tree peonies. Do some research and select cultivars who have parents from forested area's. Lacti's are grassland originated but many of the other hybrids are from forests or high shrub areas.

Review some of the earlier posts on this site for additional information and check out your local library for lending of books you might need.