Not sure about here, but in real life,
as well as the game I think, it's been awhile,
you could raise signal flags with simple phrases.
Turn left, turn right, fire on lead ship,
things like that.
So, I would think a small amount of chat is ok, but you should not be able to super coordinate anything, especially movement. (you know, go 2 hexes straight, then turn right, and then straight, and I'll go 3 straight, and turn left, and we will be in line) that sort of thing should not be allowed.

my two cents