As far as I remember I always could find dropped NPC in the same place where I left them if it was "friendly" far-a-well (Montaron, Xzar, Garrick, Jaheira, Khalid and Kivain for 100%, I never bothered to go back to the Gnoll Keep where I left Xan
). Once I dropped Branwen when I had to high rep., she went angry and disappeared - I needed her so I just reloaded and I dont know if and where she would appear. I think that Montaron and Xzar were available even if were disappointed with you.
I preferred to set my party as early as possible and then I dropped Imoen only for a while to check guys statistics and have nice talk.
SPOILER
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In BG1 I didnt find quests really depending on your actual party (like BG2). If this quest is about wywerns head you can take guy in and immediately drop him. I got reward without him in my party.
Zbyszek