In regards to the Alien cap ship it need huge nerf, way more OP per FP than any other ship and 14 large slots (defense platform has 15 but really slow) extremely fast for such a large ship, tons of flight decks. Basically it has the best OP, the best weapon slots, great flux dissipation, the best speed, and the best carrier capabilities of any capital ship all condensed into one best at everything ship.
With the exception of speed, the Omega beats the Behemoth at everything.
The Behemoth has problems:
a) Four useless hardpoints with narrow arcs in front that no sane person would ever use. (meaning there's really just 10 large slots)
b) Not even close to enough flux cap and dissipation.
c) A lot of it's turrets have near-360 firing arcs, which in itself isn't bad, but it further adds to the flux problem.
So in the end you'll either fill it's turrets with only very weak low-flux weaponry, or else it'll permanently be at full flux and can't use it's shields.
Which is also why it is extremely easy to destroy if AI-piloted: The standard weapon and weapon group setup is so bad that the AI cannot handle it and overloads itself after the first volley, thus getting destroyed right afterwards without the ability to put up shields or fight back.
The Omega on the other hand has twice the flux capacity and more than twice the dissipation. Also it's turrets have more limited firing arcs, so it is able to fully autofire and have shields up without danger of overloading.
It only begins having flux-problems if both the left and the right side of the turret mounts fire at things, and even then it is not nearly as severe as with the behemoth.
I'm pretty sure that both the Oort and the Homeworld (the remaining two "huge" ships) are able to take the Behemoth out.
Thus the only really great thing the Behemoth has is it's comparatively high speed, and I don't think it needs any major nerfs to it's combat strength.
I do agree, though, that it needs one nerf: It's campaign speed.
Alien fleets are way too fast. Weak fleets cannot escape them and strong fleets cannot catch them.
And because the rest of the fleet is a bunch of useless pieces of crap, they die very soon, causing the remaining Behemoth to soar around the campaign map really fast, escaping any possible danger and utterly destroying lots and lots of weaker fleets.
My suggestion would be to nerf the Behemoth's travel speed and then reduce the spawn rate of alien fleets while giving them way stronger other ships (1 or 2 Alien Cruisers, maybe a proper, not-useless Alien Destroyer) - that is with whoever made the alien ships, though. Otherwise just give them
a lot more FP for magisters, frigates and fighters.