Granted, these would only have first-strike capacity versus gauss cannons, but unless you're building an extremely heavy interceptor, half-size lasers are the only other option, and they have very little punch. I have no idea how you're making meson-armed fighters that are not only faster than invaders, but fast enough to close to firing range before getting blown apart by plasma torpedoes.
My last batch of meson fighters were down to 300 tons even, and that was with me being generous with the fire control's range, making sure they had 3 days worth of fuel (even with a stealthy carrier, having the range to keep the main fleet from detection is crucial), and giving them all a 0.1 size active radar for backup targeting (based on combat experience, this is unnecessary, and all targeting will be done by the pair of scout fighters attached to each squadron of 22). A size 450 meson fighter has too much wasted space, and needs to be pared down.
As for closing to firing range, I've researched on the technologies I mentioned being crucial previously: engine speed and thermal signature. 12% signature and (magneto-plasma I believe? Will check later) engines are sufficient. If meson fighters are the backbone of your fleet, you can focus solely on meson focusing, and leave the size of the beam as 10cm (same damage as a 25cm beam, after all). Your fire control similarly will need range as its highest priority (given the free 4x speed fighters get).
It's not a lossless strategy, but fighters aren't expected to all survive. That said, it's cheaper to build a few replacement fighters after a battle than to completely replenish a fleet's stocks of missiles.
They are not.
You just build enough of them.
A meson armed fighter can be around 400-450 tons, which is reasonable, but not faster than a gunboat with one meson.
It is, however, cheaper.
Fighters are also smaller, which helps with not being detected until almost in firing range, and with being a hard target for enemy fire. Nor do they require a shipyard to be built, unlike a gunboat/FAC, which is the benefit I find most valuable. The speed a "FAC-engine fighter" brings to the table is helpful, for sure, but comes with very real tradeoffs.