Well, I went that route and tried to build small 2000 ton Missile Attack Craft with the Excoriate ASM, but I ran into a major problem. Namely, I can't get useful salvo sizes without using box launchers, and if do use box launchers, I have nowhere near enough magazine space to overwhelm the point defence of my mainline fleet, even with very large swarms.
For about 29,000 BP (the cost of my fleet), I can build ~40 missile boats, each carrying 10 ASMs and 30 AMMs, for a total of 400 ASMs and 1200 AMMs. Unfortunately, I'll need to close to ~50 million kilometres to fire my missiles with the gunboats, so they need to be faster than my mainline fleet, which results in them turning into fuel-guzzling hogs with barely any range. So they're useless for anything other system defence. Even then they'll need to advance under a missile onslaught, and since my mainline fleet can carry 440 ASMs, about 200 ASMs will find their mark with the number of AMMs the missile boats have. Since the missile boats are so small, they have no shields and barely any internal HTK, so even two hundred missiles will kill half of them. At that point, the remaining craft have too few missiles to overwhelm point-defence, and what makes it through can wound maybe one ship. Then my warships will close to AMM range and slaughter the remaining missile craft.
So smaller missile boats don't work, because they're too fragile. I'll have to upgrade these to full size, short-range missile warships, but even then I won't get useful salvo size without box launchers, and using box launchers means I can't reload, which amplifies my magazine issue. So that doesn't work either. The only way I've successfully killed my fleet so far is by bringing along twice the number of ships.
Actually, since I need to build two fleets anyway for each expansion route, this might not be a problem.