Checking a collection of Ints
An alternative approach is:

require (teamSizes.min == teamSizes.max)


but ỳour forall-solution expresses better the idea, that all members share the same size.

And without measuring it, or trying to investigate my assumption, I guess that my approach traverses the collection twice, which could be a drawback for bigger collections or if performed million of times. So less so in this example.

But maybe it is possible to model the constraint into your design, so that all teams get the right size from the beginning?