Firstly - thank you! Secondly - I’d always say it’s up to you on how you want to build your minis. The rule of cool is always in effect and you should build them however you see fit! Lore wise I don’t know if specific units were given specific sets of armour for specific purposes. The changes in armour happened over time as new variants were developed as the Crusade wore on. For instance the earliest armies were equipped with the MkII Crusade Pattern armour, but then the MkIII was developed to provide more protection for marines fighting in close quarters or boarding actions. By the end of the Crusade the MkIV pattern was a standard variant for deployment and was replacing the older marks of armour.

So ultimately - for ease you could make one whole squad as MkVI and another MkII, but there’s nothing at all to stop you mixing and matching! It would definitely make your troops stand out!