New members are always welcome at Star Trek Legends. We are a growing fleet with international members and are looking to expand further. Friendly and relaxed atmosphere without any ridiculous requirements for members. We have a website and forum which are moderated and suitable for everybody.
The fleet credits you earned are yours, whether you're in a fleet or not. The only way you can lose them is to spend them.
If you were in a fleet, left it for another, then returned back to the original fleet, you would still be able to add to whatever score you had on the leaderboard as if you had never left.
Any fleet rank promotions you had received however would obviously be removed if you left and came back. To regain your previous rank you would need a fleet member or officer to promote you back to your previous level in order to have that back.
Hopefully this answers any questions on how the system works.