As others said GTVA's main firepower should be light ships, not super-uber gigants like Colossus that costs huge amounts of cash, requires 20 years to be built and can fall in one coordinated attack (even NTF could've bring him down if Bosh hadn't order forces to make run to Gamma Draconis). Shivan tech REing is quite troubleing, Sekhmet, GTVA beams and Kayser uses only small part of Shivan tech combined with GTVA one, Hades is another case, it's filled with Shivan electronic systems and must've cost a lot (remember that building it started during T-V War, prehaps GTVA has got one Hades under construction that would be released few years after Capella's destruction). Hecate was one of the largest Command's mistakes. It was supposed to be carrier and fleets' flagship, it is crap and unnessesery battle carrier with too weak AC weaponary to fight with Shivan destroyers and too small fighter capibility to be large threat for Shivans. Shortly - warship for everything, but for nothing. It can serve during next few years when the only threat are pirates with old fighters, bombers and modified freighters, but after that they must be either modified, scrapped or loaded with meson bombs.
One thing about GTVA's developement - GTVA must give itself at least 20 years to rebuild and modernize economy, research into new technologies and so on before building dream fleet. Also after some discussion we on freespace.pl realised that the only effective fleets to battle with Shivans are ones based on light forces (Guerrilla Fleet, two driffent fleets developed by c914 and me) or based on light forces supported by large capships (Battle-Guerrilla Fleet, developed by Mekton). I'll write more about them when I'll have more time.