There are barely any situations where it's good call to turn in the side guns if the front gun isn't a hwacha.
you dont turn in the side guns. the side guns gain arc incidentally as you move and twist. you are not playing for high dps in a goldfish you dont need to be hitting every time. you just need to hit at the right time. when it is of most value to your team.
If you are flying a goldfish you must fly unexpectedly aggressively and aggressively defensive. with the speed of a goldfish you can weave in get some hits and get out, not easily mind you, the engineers need to be up to snuff. you can also fly in such a way that you target the ships targeting your allies. you can remove enemies from arc on your allies and you can just straight up remove the damage sources. giving your ally time to repair and kill the threat. (this all assumes you are using the hwatcha or carronade which are the most popular weapons on the goldfish, if you are going for a damage goldfish with the flak or lumber your gunner better be good because you have to be constantly changing target to what ever ship is the biggest threat/easiest pickings.)
by removing damage sources from your allies and thus preventing damage that was going to be taken, you are essentially healing your allies, in a round-a-bout way. the goldfish functions much like the cleric in 4th edition dungeons and dragons. yes you are supporting your allies but you are doing it by beating the fuck out of the most opportune enemy. and it becomes more and more useful the more allies you have.
if you turn the gun arcs forward you remove its capability to be hitting almost no matter what direction the ship is facing and moving, thus making it boring.