This idea is awesome, I love the idea of having some sort of backup when i'm attacking something or under attack that just shows up. The only thing I would say is that I don't feel that you should pay for the backup to avoid exploitation. I know that back when you could buy blueprints i had one that was just shield and power blocks that I could buy regardless of shop inventory and being able to pay for a fleet of those just seems like a step backwards balance-wise.
Instead, I feel that it should be resource based, meaning that when a fleet is called, the closest faction owned shipyard would churn out the ships and they would make their way over to the location using jump drives if they have them. Then afterwards they would make their way back and deconstruct once they entered the origin sector returning the resources to the station. This would make calling backup a large risk of resources which I think are more valuable than credits and maintain somewhat of a sense of realism instead of having ships just appear when called for.
Alternatively, this feature could just be lumped into the rumored fleet control mechanic where a higher up member of a faction assigns AI ships to "fleets" which just linger and can be set to respond to distress. This would be cool because the ships wouldn't have to find a way to spawn or despawn because they are part of a persistent fleet and could be used for other things.
Awesome stuff so far though.