Weapons wise, the UEF has a few things to offer the GTVA:
The UEF's strikecraft missile lineup is generally superior. The only GTVA missiles that compare favorably to their UEF equivalents are the Harpoon and the Trebuchet. And that last one really depends on whether you prefer an extra km of range or extra damage. Slammers and Paveways are a given. The UEF's diverse bomb loadout is also worth looking at. The Jackhammer's a good replacement for the Cyclops, but it's entirely possible that it wouldn't fit in the internal bays of GTVA bombers. Same goes for the Sledgehammer. Warhammers seem more easily adapted, and it might even be possible to equip a modified version on fighters.
As for the primaries, it's hard to say. The Kayser and Rapier are almost identical, and the UEF doesn't really have an equivalent to the Balor, which appears to be the future of GTVA primary weapons. The heavier UEF primaries; the Archer, Redeemer, and Vajra, all have one big flaw: Ammo. More ammo-reliant weapons means more ammo to be carried by the support ships, which either means less ammo overall or bigger (and more vulnerable) support ships. Their sheer effectiveness might be worth it, or it might not. Like I said, it's hard to say.
Strikecraft. The low endurance of UEF fighters means they wouldn't integrate all that well into GTVA doctrine. As defensive craft, maybe. The exception to this is the Lapith. Bombers are already low endurance craft, and unlike the overly heavy Durga and Vajradhara, the Lapith wouldn't be quickly swarmed and torn to pieces by Shivan fighters. I don't think it would replace the Artemis, but used as an offensive escort for capital ships, striking at targets of opportunity in a capital ship battle, it could be very effective. The same could be said of the Uriel. As purely offensive tools, hitting Shivan capitals in big formations, though, I don't know.
The Kentauroi might have potential as a short range interceptor for installations and destroyers, but it doesn't have the endurance to do much else. The Uhlan has no chance whatsoever of replacing the Perseus. They're already near identical in a straight up fight, but the Perseus has the advantage of already being completely integrated into the fleet. The Uhlan just isn't worth the trouble.
Most of the UEF capital fleet is well suited to being system monitors, acting in defensive operations where logistics matter less. A Solaris could make an excellent fighter base and command center for a system. Throw a sprint drive on one, and it could also be useful to deliver a hammer blow when you urgently need more firepower, like the Toutatis in Aristeia.
On weapons, the Khatvanga (UEF PD turret) is a natural choice. It's better than the TT2, and doesn't rely on ammunition. Burst flak less so, mainly because Standard Flak exists (if you're like me and try to look at Standard Flak as some sort of newly developed rotary flak cannon that still has issues to work out. Ignore that it was on vanilla FS2 ships). For non-PD weapons, the UEF doesn't have that much to offer. GTVA torpedoes are tertiary weapons, and the Eos is generally better than the various Apocalypses except the Narayana's (which is bigger than a Supernova). Mass Drivers might work as heavy turret-mounted tertiary weapons for destroyers, and/or as replacements for every SGreen in existence. But again, ammo.