I really like (or is it miss?) parts of the Second Edition/Necromunda rules. I thought 'to hit' modifiers added to the game, as did Armor Save modifiers. Different movement rates for different units as well. The close combat was/is good for smaller engagements but really bogged down with a lot of troops fighting it out.
So we shall use the Shadow Wars rules for a kind of mini-campaign to enact the rise of the Genestealer cult over the other cults, gangs, and Enforcers. Hopefully at the end of that, 8th Edition will be released and we will jump right into the main campaign with the shiny new rules. It will be a slightly odd transition, but it will all depend on how much changes in 8th.
I do love how many factions got released for Shadow Wars right away, but I am missing most of the key ones for my Campaign. It won't be too hard to make stats for Gangers, Enforcers, and Sisters. I guess we will need Inquisitors and their friends as well. I think Deathwatch is in the main rules, at least I hope so.
Wanting to play two different rulesets in one campaign just highlights that no single set of rules can do it all. Skirmish games that have a handful of models per side tend to really break and/or slow down with larger games. Rules that can handle company-sized engagements quickly don't play as well with low unit counts. The current 40K rules (3rd edition onwards) has grown towards larger battles, but tried hard to keep small unit flavor, resulting in a bit of a bloated ruleset. 7th does play better than 6th or maybe even 5th, but a major overhaul is really needed. All will be revealed soon enough.
In the meantime we will enjoy the Shadow Wars.
The Emperor Protects
The Adeptus Rules Administratum is Hard at Work