Hang on, hang on... are we using techroom fluff to determine yield? HA!
Game mechanics are a silly thing to base analysis on (go ask any HALOites) but Helios detonations aren't deadly to even fighters outside a pretty small range. Unless we're going to inflate standard FS guns etc to ridiculous levels of firepower, it's not very consistent. They certainly aren't focussed, so roughly 50-60% of the yield will be wasted: more if it strikes a pointy bit, as on a Shivan ship. So what's the consensus on sources here? Are we just going to read the techroom and believe it, even when its wrong?
Using fusion detonations AND antimatter is textbook weapons wanking. Not only is antimatter a silly thing to make a weapon out of (god knows what'd happen if you blew a fighter up carrying such a device) but it's too hard to get proper effiency, given the low odds of getting each antimatter particle to hit a piece of matter at exactly the same time before the fusion bombs(lol) force them apart into an explosion. Thankfully, since the techroom is already inaccurate, we can ignore it!

Realistically (ho ho) nukes in space would reduce warfare to hiding as far away from everyone as possible and trying to catch someone in the killzone of a detonation.