That Zeus class is a thing of beauty. A potentially Intellectual-Property infringing thing of beauty, (as I assume it's being built and sold for profit without GW permission) but still a thing of beauty.
The Orion Fast Clipper from Into the Storm is a transport, not a modified Sword Class. But for modelling purposes it's pretty close to a Sword class in looks.
As for the old Spacefleet ships, I still like these. I like the old plastic Imperial ships, which were called Gothic Class cruisers. They have a totally different (but, I think, still very "Imperial Gothic") look to them. Where you'd fit them on the "Old, pre-Imperial ships (a la Emperor Class)-Heresy-era pre-prow shipsancient intermediate Grand Cruisers-"modern" prowed Imperial ships continuum is a bit debateable.
From memory, the Imperial spacefleet ships were:
-plastic Gothic cruiser (without the "modern" Imperial prow)
-metal Cobra Destroyer (without the "modern" Imperial prow)
-metal firestorm class cruiser (without the "modern" Imperial prow)
-metal Emperor Class battleship (with Imperial prow)
-metal Ironclad class battleship (without Imperial prow)
-metal Tyrant class battleship (with prow)
-metal Dictator class battleship (with prow)
-metal Thunderbolt class cruiser (without prow, quite different to most Imperial ships)
-metal Castellan class shield ship (no Imperial prow, very un-Imperial looking)
-metal galaxy-class troop transport (no Imperial prow, very un-Imperial looking)
some sort of escort ship, whose name escapes me (Stalwart?)
the Goliath class factory ship (very un-Imperial looking)
and some sort of sensor ship, whose name totally escape me...
There were also two very specialist Imperial ships in metal that looked totally different to the others: one was (I think) called the Annihilator, which was equipped with massive cannons in a revolving turrent that took up most of the ship's length, and the other was (I think) called the Dominator, which was a bizarre looking specialist boarding craft equipped with two gripping arms and a revolving drill bit. This is all from memory, I don't have my White Dwarf collection on me! There were also Eldar ships and Tyranid vessels, including legendary non-Tyranid "Kraken" hijacked by the hive fleets.
It would be interesting to have a go at statting these things up. If you're scaling them on the same basis as the later Battlefleet Gothic ships, they'd be a bit smaller than their equivalents, just because the miniatures were all a bit smaller. The Spacefleet battleships are all BFG cruiser-sized (if that), and the plastic cruisers are light cruiser sized.
You could rename these vessels to anything you please to fit in with your Rogue Trader game, or you could just say that they are the same classes as the later BFG ships, just made in different Forge worlds. That's the approach FFG have taken with the Fury class-Interceptor in Into the Storm: They say that the "class" designation just describes vessels made within certain parameters, and that they can actually look very different in practice.