In the original Might & Magic, is it optimal to go with just one 6-character party, or is it better to have some extras to swap in and out for class/gender restricted areas? I've mostly been exploring/grinding in the vicinity of Sorpigal so far, but I notice at least that the town of Portsmith seems to hate male characters, for instance.

Six is enough.
I didn't have much problems with the default party of only one female, but I skipped most of Portsmith.
If you create new character, I recommend a gender balanced party.
I think there's also some monster that can only be hit by either males or females too. Defintely was in MM2, but not quite sure about MM1.

Six characters is enough for MM1. I wouldn't blame you for fielding an entire party of girls to investigate Portsmith, but I believe there's a fountain or suchlike in the caverns below the town that can swap the gender of your characters, so you can use that.

You could use a seventh character to store items for you at the inn but that's not necessary.

I think six characters are completely fine, if you want to have another character just in case, it will be fine, but I reckon you will encounter issues with leveling him/her up.