I usually don’t comment on the Historical customs (since I’m usually not well read on them), but Nagle looks awesome! I agree he could be very powerful in a game, and combining him with Commander Temple could get messy in games with a point limit (basically anything not a campaign). I think the way you’ve worded the ability is fine, and 5 gold I think is fine, I mean Barbary Banner is essentially a 5 coin to any non-corsairs.
Those mercs are very interesting. I like the Starfire, I experimented with that on a few pieces from SpMg. Her base move and cargo are nothing special, but that’s good, since her main ability is devestating, not even a high point cost could keep it from being OP or broken. This is one ship you’d HAVE to get the first shot on, or have a canceller handy.
Harrion is cool, I like the combination of Captain and re-roll, and the grapple shot makes him quite intriguing. With that you could put him on a sniping ship and go nuts, or combine him with WK Nemo for even more fun.
Stormguard sounds like it came out of a fantasy novel, I love that name! Anywho, nice little fort. A decent defensive ability for a fort, and allowing Harrion to use his abilities IN the fort is interesting and cool.