If we are going to have t3 and t4 fighting t2, at least allow the people who joined as recruit and have to fight t3 and t4, be able to pick from their t3/t4 ships and so on. They've played the game, unlocked the ships, bought them with whatever currency; there isn't a good reason in my mind to lock them out while they have to fight vs those very same ships on the enemy team.
I think people will eventually start leaving games to try to get more favorable ones. I'm not the kind of player to do it, but I could easily see someone going 'I'm not going to play my t2 vs t3 and t4 screw this' and leaving a game in progress just to reselect their t3 or t4 ship. I don't want a spiral effect of people leaving games and joining them and getting used to the idea of flipping through games to get one they could have fun in / constantly joining a lost game in progress, on the losing side as if the 'join game' system were a slot machine. Currently, anyone who has t2 and t3 unlocked and joins a recruit only to see t3 and t4 on the enemy team, has every incentive to leave the game and join a new one under 'veteran'. Given a choice people will likely pick; they only have to come to a level of frustration with it to make the choice.
If we're going to be adamant on allowing t2 and t4 into the same match without smoothing the performance gaps (read: rebalancing), lets allow players in mixed tier bracket matches to pick from their available tiers that are available to that match. Its bad enough to have t2 fighting t4, but to lock that player into playing t2 when they have t3's they could be using is locking that player into a bad game experience they could simply avoid by leaving the game and rejoining.
I see a lot of this abandonment of 'lost cause' games in overwatch. 6v6 becomes 11v6 where the 11 never had a full team at any point in time, people just kept joining and leaving, and it was never a fair game. This feeds into negative attitudes and toxicity, and players who dont understand that the match was decided by the matchmaking system turn on other players as the reason for failure. There almost isn't a session of overwatch I sit down to where somebody makes a negative comment about joining a lost game in progress on the losing side.
Rebalancing would be a big order at this point I would imagine, perhaps a shorter path to smoothing out the differences that tiered performance enhancements + the matchmaking system would be to not lock players into lower tiers when they have higher tier options available? I'm really not sure why we would disallow a player to fly the ships they've unlocked in the first place, and pick from a lower tier of options to fight the very same ship they are locked out of playing because its 'too high of a tier'.