As a general rule newfs get alongwell in large numbers, the thing i worry about is the excitement and level of play, LOL
if you have intact animals, challenging each other, that is where i might worry, so far mine have gotten along with every dog they meet, except a rotty here and there with a very strong personality, or other breeds that might show aggresion.
for instance, at agility there is a BC, it is a very insecure animal, watching her interact she doesnt know her place, her smile at first i thought was just that, but it was not, she kept turning to look at penny, and the smile was there, and a half mast tail wag, penny tried and tried to make friends thru the whole class, towards the end of class the BC challenged her baring teeth, and of course penny stood her ground.
that is the type of thing i would worry about.....
examples such as that.