If you were able to start matchmaking with a rank 8 ship and a rank 13 ship, then that’s a bug in the matchmaking (maybe destroyers are not checked properly). Normally, all ships in your hangar must be within a 3 rank range. 8 to 13 is clearly more than 3 ranks difference, which is not supposed to be allowed. You really shouldn’t be able to even start a match with rank 8 and rank 13 ships together.
EDIT: That 3-rank-range is only checked for squads, not for solo players. The game matches your biggest ship to fit within the match range, and everything that is below is denied access once you enter the match briefing. So it may sound wrong (and it really is), but currently, this is a feature, not a bug. Alternative suggestion see spoiler below.
As I described [here](< base_url >/index.php?/topic/29764-stricter-restrictions-of-ships-when-squads-are-joining/), that approach is questionable, as it imposes a hard limit. And nobody wants more limits - the game already has way too many of them. Team composition is the responsibility of the matchmaking, and ONLY the matchmaking, not of the match itself. All ships that have been accepted by matchmaking must be playable in the match - that’s the whole point of matchmaking!
There is a compromise without butchering the software architecture: matchmaking refers to the highest ranked avatar just like it does right now. But instead of hardlocking people’s ships, it allows people to play the lower ones if they really want to. This way, we have less limits, and more power to the player, who can decide for himself if bringing a rank 8 destroyer to a rank 15 match is really a disadvantage for the team or not.
As for your question why you can’t play rank 8 ships in a battle with rank 15 ships, this is due to the way the progression system is designed. But that’s a matter of game design and has nothing to do with this topic.
It’s an old leveling philosophy that basically “encourages” us to spend alot of time in the game (some call it grind), because higher rank ships are better in everything than lower rank ships. Except when they aren’t, like with destroyers.
The matchmaking accounts for this player progression system by allowing only people to play together who are nearly the same rank - otherwise there would be a very unfair advantage for the higher ranked ones. With enough players, the MM can result in matches where literally everyone is the same rank (if the game had representants in every rank), leading to theoretically balanced matches. See World of Tanks as one of the prominent examples. There may or may not be additional MM variables like player skill. That depends on what the respective game developers want.
Of course the whole thing starts to fall apart when you release ships that are much stronger than everything else on the same rank (or everything else in ANY rank, for that matter). Because the whole thing is based on the world-holding pillar of “time progression equals avatar strength”. And when that is no longer true, inconsistency increases and weird things happen.