I think it depends on what exactly you are trying to achieve.
While I understand the interest in having a merged install, I think it presents all kinds of challenges. I think generally people run era/campaign/theater specific installs for actual gameplay. It can get dicey mixing era, weapons, etc.
I have installed several terrains and aircraft into WOI. Terrains work, A/C are sort of hit or miss. I think that is because of the changes/upgrades made to WOI.
Some modders are starting to release A/C mods which have been upgraded to WOI standards.
I have decided to hold off on any installs, merges, etc. until after the patch comes out that brings all TW sims up to the WOI standard.
Just my $.02
And yes, it does help to RTFM, several times. And there is one for merged installs.