I don't know if it helps you with your own marines, but here's how I divvyed my
40k Salamanders armour up:
1st Tac Squad: 6 Suits Mk 3, 4 Suits Mk 4 (Destroyer/VHA variant) - Explanation: I like to imagine that the first squad of each company is sort of the 'Shock' squad, comprised of the companies senior marines and tasked with Zone Mortalis, Boarding and Close Range Firefight duties.
(Also usuable in games of 30k

)
2nd Tac Squad: 4 Suits Mk 3, 6 Suits of Mk 5 (Mix of components from Mk 4,6,7 armours) - Explanation: As Salamanders are techno-masters, they'll have far more 'relic' components in this rank. So the 2nd Squad demonstrates this by having a mix of
HH and
40k components mixed up.
3rd Tac Squad: 1 Suit Mk 8, 1 Suit Mk 4, 2 Suits Mk 6, 6 suits Mk VII - Explanation: Your typical squad with a mix of newer Mk 7 and Mk 6 with a relic Mk 4 and a rare Mk 8.
1st Dev Squad: 5 Suits Mk 8 - Explanation: I like the idea that the Devastators got all Mk 8 as the advanced systems aid them in laying down accurate fire and provide their exposed position with extra protection.