Well, I don't know much about modding but....
I would assume you are better off using an autocannon weapon to shoot at strikecraft (for TEC at least). It looks better, and is more of TEC's style.
You need a weapon fire effect, a weapon projectile effect, and a weapon impact effect for a weapon to have visible FX and whatnot. I don't think the effects used for strikecraft are different from frigates/cruisers/capitals but if they are you need to add them in. A nice flak-style effect would be the one from Homeworld 2 with the explosion and the lingering black smoke. You should add AoE damage as well because flak explodes. Updating the Garda into a Homeworld 2 style flak frigate would be cool.
Oh and a mod you should consider using in your mod is the SolarFlair mod. Its even better than Lost_WLD's graphics mod in my opinion.
SolarFlair for Sins of a Solar Empire The Advent Plasma effects kick total ass. I'm trying to nag the maker to put up a version 1.0 vid. But he's dead maybe.
Anyways, ignore my mindless rambling if you have to. You should still consider using SolarFlair for your mod. And having the Garda shoot actual flak would be sweet.