I would be most concerned about the peony roots. Clear off the existing soil and bring in some that is uncontaminated for the peonies. You don't want the preen getting down by the roots which will be planted shallowly. Preen can cause some root pruning, or in other words, slow down growth of roots, and now is the time you want those peonies growing the new roots.
Preen is marketed as an inhibitor to seed germination, but you don't want it touching any roots, and peony roots may be shallow, or you may be stirring the preen deeper when you plant. Does that make sense?
The best way to handle new beds is weed them, (till or kill) then plant, then spread the preen.
Maybe Polly will come on here and advise you on the siberians. The bulbs will be fine.
