For your front weapons you would be better served picking weapons that compliment each others function rather than overlap. Flak Cannons do explosive type damage, which is great for finishing off an opponent who's armor has been taken down, but will take a long time to take down that armor in the first place. Gatling guns take down armor, but struggle to deal hull damage. Pair these two guns up and you have one the most stable and consistently useful weapon combo's in the game.
For side guns take whatever you feel comfortable with, but a gun with good rotational firing arcs are ideal on the first port side weapon, cause with some careful flying you can potentially line it up along side the front two guns and form a three weapon trifecta.
For back guns go with something with utility (beacon, for example) or something to dissuade pursuit, such as an Artemis or Dragon Tongue.
The Manticore on a Goldfish is very, very good at burst disabling an enemy ship, but the long reload makes actually delivering a death blow difficult. You'll need to bring you side guns to bear, realinging the front weapon only when it's ready to fire. Or have an ally finish off the crippled enemy. The Goldfish does well with any heavy weapon, but the Manticore is the most common, followed closely by the Hellhound.
Galleon is all about having good gunners. Practice with the different weapons. You also might do yourself a favor by putting all long range guns on the port side (Mercury, 2xLumbers or Flaks) and Manticores on the starboard. Try to fight from range as long as possible. Get into position fast and then hold the ship steady, as shooting at range from a moving ship takes a lot of practice. In my opinion though I would avoid the Galleon with an inexperienced crew. Even though it has a ton of durability it's far from invincible and requires a lot of knowledge to make the right calls, as you often need to anticipate opponents actions and positions. It's cause the ship is so slow, you need to be moving before it comes too late, and the window is very small with the Galleon. Also the heavy weapons tend to be harder to use in general (except for maybe the Hellhound, which isn't a very good pick on the Galleon anyway).
Hope this all helps!