Is a rudder really a must if I'm paddling with other rudder equipped kayaks?

It really doesn't matter who you are paddling with; you want your boat to go in the desired direction with a minimum of fuss, even (or especially) when paddling solo.

It depends entirely on the way your boat behaves in wind and waves. If your boat needs a lot of 'course correction' strokes, it can burn up a lot of energy that can be put to better use propelling the boat forward, and a rudder can help.

In my limited experience there are different kinds of 'rudder boats' - boats that need the rudder to go straight ('track') and boats that need the rudder to make them turn. And there are boats in between those extremes.

Many boats can be made to 'behave' in windy conditions with a skeg.

IMO, a rudder is usually easier to retrofit than a skeg, and you don't lose cargo space with a rudder.

Perhaps other owners of your boat design can give advice, and the designer (Nick) should be your first source for information.