When doing regular story missions, none of that really matters. For STFs you need a team of 5, you can't even start one without a full team. Same is true for the other missions in the PvE queue.
As far as the members of the team is concerned, it doesn't matter much. I've done STFs with all Cruisers, all tactical captains, sometimes 2 science, sometimes 1. No matter what, your team can be successful.