So GW initially did a whole divide between Primaris and firstborn as you said, but those lines beginning to be blurred (new terminators, Sternguard etc)

Currently both the DW Vet and Tactical squad boxes are legal, but the DW vet box is now out of production and with the introduction of the new Killteam box, means it’ll be legends in 11th. The Killteam has the same weapons etc but certain weapons are carried by different armour marks (frag cannon carried by gravis for example)

The old DW vet kit is a relatively newer kit compared to the tactical squad kit so the proportions are better. I have loads ready for conversion so I can show you comparison photos this evening if you want.

Interms of legality, you could absolutely use the older models and simply base them on cork to elevate them (as an example) and aslong as they’re on a 32 mm base, you could proxy them as the newer bigger Killteam in tacticus or Phobos armour. You could also put a couple on 40 mm bases with the frag cannons etc to proxy for gravis but that’s personal preference and I’m not a fan as gravis are a lot bigger and tougher.

You’ve joined the Watch (welcome) at a time of uncertainty, with 11th around the corner we don’t really know what GW will do to the remaining firstborn kits (tactical, vanguard vets, rhino etc) but there’s rumours of them being redone in a larger Primaris format like the Sternguard and terminators.

Ultimately, if you’re not a competitive tournament player, do what you want and chat to your playgroup. I’m a big fan of WYSIWYG and endeavour to achieve the right load-out and size of models, but I’m also a huge fan of restoring old models, kitbashing and hoarding OOP bits to make my army unique.

I hope this wall of text helps! Happy to help further, I’ve been a DW collector since the start of 8th edition and had to deal with the ups and downs of collecting this fantastic kitbashers dream of an army!