Just a quick question before I boot up TOEE with the Co8 mod pack installed. Is it of any use to read the TOEE manual from gog considering all the changes Co8 makes, or should I look online for new documentation?

I haven't read the manual in a long time (haven't installed the GOG version yet, so haven't looked at the manual again), so can't remember how comprehensive it is, but it may well be worth it to brush up on the basic game mechanics as well as some background to ToEE itself. Then have a peek at the Co8 changes to get yourself up to speed.

Actually, the Co8 doesn't change the base mechanics of the game much, and those are precisely the things explained in the manual. So yes, you should read it.

Also, be aware that some information can only be found in the manual (the list of what bonuses familiars give for instance) and some others can only be found in the in-game help (like cleric domains descriptions).

Taking the time to read that stuff helps you make informed decisions when you create characters.