Its a great Q. As the leader of the group you would be the visionary and have outlined your aspiration to have your team on a unified code base. I would suggest you involve the team in a brain storming session(s) to discuss the stacks and what the pros/cons of each base are for each of them and as a collective, continue to probe each individual and then propose your opinion that 'sound like we would be a lot more productive/efficient working on a single code base' and lead that conversation into the one that majority agree on. But this may take a couple of sessions depending on how irritated some individuals get by the idea of 'letting go' of their preferred base. Goodluck.