TDD rewrites Irenicus' character in some pretty major ways. That alone is reason enough to stay away from it if you ask me.
Late edit: I just realized that I got The Darkest Day and Tortured Souls confused here, probably because they both have similar silly names and add some BG1 companions.
So, Tortured Souls is the one to stay away from because it rewrites Irenicus (and that's definitely not the only reason).
Although I don't like The Darkest Day either, since it changes the rules (spell progression tables and stuff like that) and just isn't very good in general; a lot of it is just boring filler material.
If you want BG1 NPCs in BG2, I'd make a Baldur's Gate Trilogy installation with the BGT Tweak Pack installed, which has a component that allows you to port all the NPCs that don't appear in BG2 (which means no Safana or Xzar, but you'll get Yeslick, Branwen etc.).
Of course, some companions already have mods made for them, sou you may not even need the simple port option in some cases. You can use this thread
over at Spellhold Studios to look that up.
(Obviously, some mods have questionable writing. I'd stay away from the Kivan and Deheriana mod simply because it's done by the same modder that wrote Kivan for the BG1 NPC Project, which was a complete character assassination, amazing when you consider how little character BG1's NPCs have to ruin in the first place.)
And if you want an NPC that you can't port or get a dedicated mod for: making a Baldur's Gate Trilogy installation means all the files for those NPCs can be accessed in Shadows of Amn, so you can always just cheat them in.
Oh, and there's also this mod here
, which appears to be in active development.