Superdestroyers: Man, why are we still having this discussion? I already said it's too expensive to be a viable option.
The Sathanas in AoA doesn't invalidate my point, it proves it. The Sathanas's initial appearance allowed the Shivans to destroy the Keeper, putting all the Vishnans out of action and forcing the GTVA into retreat. Would three Bellerophons have been able to do that? They might have destroyed the Keeper, but not as quickly and they could easily have been destroyed too. A shock jump by a supercapital, on the other hand, will often instantly win the battle by completely ruining the enemy's plans. This is partly because of the immense firepower supercapitals have, but it's also because of the emotional impact that such a huge ship produces. True, the Sathanas was later destroyed, but that's because the Shivans made the mistake of giving the enemy enough time to react to it (IIRC, I haven't played AoA in a while). Used properly, a supercapital could conceivably shock jump an entire battlegroup and get away with it.
The Vishnan Sacred Keeper is as big as a juggernaut, but in terms of armor and firepower it's midway between a destroyer and a supercapital, making it a poor example. As I said before, the Colossus got destroyed because its designers (and Command) had the arrogant assumption that no warship of comparable, let alone superior, size or firepower would ever exist.
True, a supercapital is Sathanas bait when inactive, but so is a destroyer. Similarly, shock jumping a destroyer is much easier than shock jumping a supercapital. Every single flaw that applies to a supercapital, with the exception of extreme cost, also applys to destroyers, and usually to a greater extent.
Hecate/Solaris:
Solaris/Hecate: take a look at the size of the Raynor/Titan TerPulse turrets. They're huge. The Hecate has turrets big enough, the Solaris most certainly doesn't. There isn't enough physical space on the ship to fit TerPulse, and making some would add even more time and effort to the refit. Mounting BGreens would also be tricky because it requires you to add a large amount of heavy-duty power conduits for those beams, so you couldn't have many. Weapons, even those that look small from outside, have a huge amount of internal components. They're like icebergs; what you see is only a small part. You just don't seem to get how extensive this refit would need to be. You're turning it into a whole new ship. Build more corvettes instead. They're a better use of those shipyards.
The Raynor has one or two fairly small TerPulse turrets, and the Solaris has many big Gattler Turrets and Point-Defense Turrets. Replacing a few Apocalypse launchers with BGreens (or, possibly, MjolnirBeam#Home) would allow it to mount beam cannons.
I think you're overestimating the amount of space a BGreen would take up. The GTSG Mjolnir, despite being no bigger than a freighter (whose size pales in comparison to the Solaris), can mount the MjolnirBeam#home, a fast-firing beam cannon with more DPS than the BGreen or the BBlue. Not only that, the Mjolnir can still function if those ring thingies are destroyed, which makes its beam weapon actually not that big. Now, we can probably assume that, with the BGreen's lower power (in terms of DPS and refire rate), it takes up no more space than the MjolnirBeam#home, if that. You could also remove some of the Solaris's armor (not a huge loss, considering how much it already has) to make room. Or, you could take out some of its Burst Flak, since in BP canon flak guns require large ammunition bunkers. Even if it only had, say, 4 BGreens and 6 TerPulse, that would significantly increase both its firepower and its endurance. Now, it would still have the disadvantage of short weapon range, but within that range it would be a truly terrifying opponent with much greater endurance than before.
The Hecate can at some angles fire with both of its bottom TerSlash, but it can only hit with one of them to the sides. The Solaris can hit with several Apocalypse torpedoes and Gattler Turrets at almost any angle.
These would be pretty extensive modifications, but they would still be much cheaper than building an entirely new destroyer or a new corvette team (unless it was a Deimos team).
ECM:
Yeah, the Shivans might have ECM. Although, considering that the alternate-universe Shivans had the same technology (somehow) of their retail counterparts, I find this unlikely. Of course, the GTVA also used very little (if any) ECM in retail (unless AWACS counts as ECM), so that doesn't neccesarily prove that the Shivans don't have ECM. ECM would, however, be an entirely new strategy for the species that has rarely ever used anything other than purely offensive tactics. But it doesn't really matter, since the GTVA's beams can also be jammed with ECM, and we've seen that happen much more frequently than the UEF's torpedoes have been jammed, which suggests that the UEF is better at ECM (and guarding against it) than the GTVA.