I know with Ultima VI you MUST have one party member for a bit to get one of the runes. Not sure on VII though.

Ultima 7: Yes, with a few brief plot-related exceptions in The Serpent Isle.

To be honest both games (more so with Ultima 7) party members just does not feel right, Its like in a way this game was originally meant to be a solo character, but with party members tacked on. Did you felt the same playing both games, because they sure are not Baldur's Gate party system thats for sure. So is there also a way to ask a party member to leave in Ultima 6? How do you do that (and excluding just getting them killed and leave them for dead)

No, the party members felt right - they're more animated in Serpent Isle than in Black Gate, but it must also be borne in mind that this sort of party structure was actually quite advanced for the time.

(And unlike BG, they aren't prone to wandering off in completely random directions because of the terrible pathfinding. I HATED that about BG...)

In the case of Ultima 6, you can remove anyone other than Iolo, Shamino, and Dupre simply by asking them to leave. Those three will not voluntarily quit the party, so you will have to get them killed.

As for Ultima 6, I don't think that party members is a hassle - except when it comes to the dragon egg mission. But then I was teens. And I do plan to replay it these few days.

Anyway, other than buying swamp boots for everyone, and give them blade weapon, not much to worry about other than food - but they even automatically eat those.

U6 - having party members equal to more hits in a turn.