Are party members expected to be disposable and swapped out when I feel like it or they die?

As originally designed, it was expected that players would lose party members on a regular basis and need to replace them. This is why there are so many NPC options, why they become available at various points throughout the story, and why so many of them are thieves (the game is much harder if you don't have a thief in your party). Turns out, most people don't play like that: they form a party early on, and do their best to keep it intact throughout the game (this also generally gives you a stronger party). This is why Baldur's Gate II has fewer NPC options, and heavily front-loads your access to them.

If you turn someone down when they offer to join your party, about half the time you can return later and ask again. The exceptions from the original are Dynaheir (and if you refuse her, Minsc leaves), Yeslick, Branwen, Jaheira and Khalid, Montaron and Xzar, Viconia, and Shar-Teel. I don't know about the NPCs added in the Enhanced Edition.

If you remove someone from your party, they will usually stay where you left them. You can then talk to them later and invite them to re-join your party. I've seen reports that if you leave them alone long enough, they'll be "reset" in equipment, location, or both. It's not clear if this is a bug or not.