Is it possible to assign a dedicated IP address to one domain only in a subscription so that other domains are assigned to a different IP?
No, it is not possible to assign a dedicated IP address to one domain only under a subscription. The dedicated IP is assigned to the whole subscription.
Such functionality is yet to be implemented in Plesk. You may want to vote for this feature on Plesk Uservoice portal. The top-ranked suggestions are likely to be included in the next versions of Plesk.