I wished I could help some more. Well, the main thing is to keep FSX in place, so you can always fly the old sim in case the new one stutters. The installations should not affect each other to my knowledge.

As for the addons, I think some of them may work with the user-to-user help. Avsim forums may be a good idea as they seem to collect the most common solutions these days (http://forum.avsim.net/forum/436-the-unofficial-prepar3d-forum/). Other addons will work out of the box, like REX-E, which just needs an extra code to allow for P3D to work.

Some remaining ones may have trouble on the licensing  :-/ or some modules failing so far. Take some another company airports for example and their AES Lite components like the small vehicles around the place. Perhaps check if there are separate P3D installers for your purchased software. In the case of another company airports, they've updated their installers some time ago. So perhaps download them again to install them in P3D too.

I want to have:

1- REXE
2-Fs2Crew
3-RC4.3
4-UT2
5-UTX US
6-GEX US/CA
7-FSFD

REX-E needs an extra code and then works.

Quote:

REX Essential was specifically created for FSX as well as Lockheed Martin’s Prepar3D. This section is for the user that would like to use REX for FSX AND/OR Prepar3D. Purchasing a REX/Prepar3D license  will ‘unlock’ all the features specific to the Prepar3D platform, and will be confirmed by the field being

FS2Crew. So far, only the 737 thing works. http://forum.avsim.net/topic/373558-prepar3d/

RC4.3. I don't know. Maybe this helps. http://forum.avsim.net/topic/382754-two-installations-on-same-machine/ Looks like it's working with small adjustments.

UT2 may work with tricking, UTX and GEX too, but those are the ones where the companies have trouble with the license. Stating that openly.

What is FSFD?