Firstly, I'll make the assumption that you have TDM installed in a directory called TDM or similar rather than overwriting the base data files (which would be a bad idea)
Next:-
If you install P&N 2.3 it will *NOT* overwrite your TDM mod (disclaimer: unless you install it into the same directory which would be a bad idea)
when you download the zip file, extract the contents (keeping folder structure) to your SE4 directory.
This will create a SuicideJunkieMods directory under which will be Pirates and nomads.
Now, my recommendation is you 'Cut' the P&N directory out of SuicideJunkieMods and 'Paste' it into your SE4 directory, then delete the SuicideJunkieMods directory.
Sometimes it's nice to keep all the mods ordered like SJ does. Indeed I do too, but in this case when you only have 2 mods I dont think it's needed and it'll just complicate things.
Next. Download my Mod Selection Utility (the link is in my signiature)
The readme.doc explains how to use it but it's fairly easy to understand anyway.
Using my Mod Picker means you wont have to continuously edit the path.txt file to change which Mod you want to run.
By doing all of this you have 2 completely separate mods. This means the TDM races will *not* be available in Pirates And Nomads.
SJ will need to explain about his AI patcher as I know nothing about that.
I think I have covered everything.
If I haven't answered anything correctly / completely then just ask again or call me incompetent
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The Unofficial Space Empires 4 Mod Utility V2.0
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Download as a .ZIP <
HERE>
Download as a .RAR <
HERE>
Download as a .ACE <
HERE>
Check a Screenshot of it<
HERE>
Check an Alpha Shot of my Mod Editor (Unreleased) <
HERE>