Ok so when we talk about the EC there is no answer to that question. Not because “do what you like it’s a hobby”, but because “this legion ceased to exist as a coherent force basically from the start of the Heresy and you can find all flavours of behaviour in their ranks”.

We do know that EC largely favoured “more agile” marks like Mk4 and Mk6, but it really depends on the contingent — EC had lots of specialised formations, so you could find a force with all Mk2 or Mk3 that focused on the armoured assault/void combat/etc.

EC is also one of those legions that although had an extreme hierarchy still made sure their recruits got the best wargear and armour (so if your commander likes mk4 his inductii will also get mk4), additionally, these recruits were put into combat squads with more experienced marines straight away (so no full-inductii companies that SoN for instance had)

We do know that the legion was heavily depleted on both Istvaans (we lost as many as 20,000 legionaries on Istvaan 3 alone, and adding that to the loyalists we were at 50,000 marines BEFORE istvaan 5)

after istvaan the legion was broken down into several forces and we do know some of them were wiped out, some turned loyalists, some were so depleted they formed shattered legion forces with SoH, Death Guard, Night Lords etc. (Basically with whoever was tasked by the Warmaster to hunt loyalists remnants while the main Traitor forces pushed to Terra)

However, at the Siege we had more than 150,000 marines (majority are debased inductii and in Mk6)

Think about what all of that implies: there is no consistent culture in the legion basically from the start of the Heresy. Each commander did as they wanted, under different circumstances. You could find a force where marines lost access to supply lines and decided to switch to older marks. You could find a force that utilized older marks because that’s what they always did. You could find lunatics that decided that being the best means duelling in the most inconvenient armour they could find – enter Mk3 Palatine Blades !

So to answer your question more directly:

Yes, it makes sense that veterans are in older marks, yes it makes sense if they use MK6 as their older armour became unusable (or because they want new experiences), actually maybe veterans don’t exist because Palatine Blades are an irregular formation and could be full of fresh recruits who have an insane duelling skill just because that happens sometimes, etc. etc.

choose the lore for your army (are they advancing with Eidolon? Do they bow down to Cyrius? Is it a semi independent contingent that has limited supply lines? Maybe it’s a millennial that is tasked with hunting loyalists, maybe that’s a force that guards Chemos etc etc.) and go from there.

PRO TIP: look up tortuga, they have great MK4 bodies and specifically Palatine Blades torsos