It would be good to see a bigger group at Paignton. But like many Primates, Mandrills don't always take kindly to incomers and it can cause problems within a group when babies start to arrive. EEP recommendations for the Drill- a very closely related species, advise Zoos intending to start a group to use related females only, as otherwise a dominant female may attack and try to kill the young of subordinate (unrelated) females in order to further her own genetic line, a problem which doesn't occur in e.g. a mother/daughter(s) combination. Of course this isn't always possible but I don't know if Paignton may experience this problem in the future with their females coming from different sources.
It didn't seem to be a problem when Chester added new females though, possibly because two female 'subgroups' may then have been formed.