Yes, all the POSKY stuff will work in FSX except some of their panels.  Just alias them to the default cockpits and enjoy.

Yes, just change the panel.cfg folder to look like this:

[fltsim]
alias=B737_800/panel

and the sound.cfg file should be changed to look like this:

[fltsim]
alias=B737_800/sound

Note that you just have to change the _400 to _800, so it is easy to quickly go through all the aircraft to change them.  I suppose an alternate method would be to copy the sound and panel folders out of the 737_800 folder and create a separate 737_400 folder.  It would accomplish the same thing with less hassle I suppose.

If you created a separate folder to put add-on aircraft in like I did, be sure that you create that new 737_800 or _400 folder within that folder.  It took me a few days of scratching my head before I figured that out and my POSKY aircraft finally showed up in FSX.  Ay....