How many peonies/tree peonies do you have? I would recommend one bed for all of them and only for them --- you can make arrangements the way you like but tree peonies are getting increasing higher as the years wear on whereas peonies stay at more or less the same height all the time, I would suggest centering with tree peonies and surrounding them with peonies. You need to take colours into consideration too for the arrangement. One technicality you have to pay attention to: in transplanting peonies, try to level them at the previous level (there are usually red marks just above the ground) as much as possible. Planting them too deep/not deep enough may cause them not to bloom for up to 5 years depending on the soil because their roots need to get settled and they settle very slowly. Timing of transplant: the earlier the better so that roots have a longer period of the year to settle before being frozen.
Do it in the early morning and keep on watering thoroughly for 2-3 weeks.