I have a mildly crazy in-universe
theoryexcuse, that the Myrm was designed to be a sort of stop-gap anti-Lucifer measure, combined with a space superiority fighter project. Instead of relying on expensive, slow, vulnerable assault bombers to take out a Lucifer's reactors in subspace, make a new model of moderately-quick fighter capable of delivering a Harbinger/Helios class bomb or two, in a pinch. Combine/justify its funding with your new space superiority fighter project, so you can have these fighters already waiting wherever the next Lucifer shows up.
Note the oddly high hull rating(290), beyond most assault fighters. Hint that it's meant to operate in subspace without shields? In subspace, its six guns would give it the ability to brutally swat down almost any (shieldless)Shivan fighter, much as the old Herc Is did back in the day.
Do I think a Myrm really should be able to use Helios bombs, just like any other secondary, in the way it does?
No, but perhaps it's relatively easy to add some kind of extended hull or bay, and/or perhaps they have cut-down bombs of Helios-class(perhaps with no propulsion system; inertia-only), and/or perhaps it's possible to mount them externally.
As far as being a space superiority fighter the rest of the time? I'd figure numerical superiority, the Myrm's heavy firepower, and superior GTVA training would let them crush most pirate and minor rebel groups with ease. After the HoL's demise, it probably wasn't until the NTF rebellion that the GTVA had to put down a fighting force of truly comparable strength.