I don't know the exact relationship of the Pact members before they held hands & came together. The common thread between them is a dislike for the Federation & the Klingons. I can imagine the Romulans leaving the group over something with the Breen, since they were on opposite sides during the Dominion War.
said we're in 2382 now so it could take a while till we get to the events of Countdown. It has taken almost seven years to get from Nemesis to the current time in the novels (longer for DS9 to catch up) so I could be a bit of a wait.