You can add DLCs any time you want.
Point Lookout, Zeta and Pitt add new areas, which will not affect your gameplay at all untill you visit these new zones.
The last one however adds new enemies to the base world aswell. New, powerfull enemies, which will take nukes to the head while still frying your butt.

Luckily those only spawn after you reach a certain level, and sometimes in certain places regardless of level.
Just a note.. [url="http://www.fallout3nexus.com/downloads/file.php?id=1827"]This mod[/url] increases the damage of headshots, both on enemies and you. It speeds up the combat, and reduces the retarded overpoweredness of the enemies mentioned above. I strongly recommend getting it, but its up to you
[url="http://www.gamebanshee.com/forums/baldurs-gate-ii-shadows-of-amn-9/guide-to-tactical-mods-spoilers-116063.html#post1068546"]BG2 tactical mods guide[/url]
What? You're still here? Go write a review![url="http://www.gamebanshee.com/forums/rpg-user-reviews-118/"]Here[/url]
Insane Ironman BG2 let's play! [url="http://www.bay12forums.com/smf/index.php?topic=81201.msg2140894#msg2140894"]Here[/url]