Mine only swim together when they are scared. That is when I add new fish(rare) or when I do the water changes or move plants. They are not shoaling fish if that's what you mean.
my mollies did. i used to have a black molly and a 24 karat molly, they would swim togehter all the time. then one day the 24 karat molly died. soon after that, the black molly died of dropsy.