Your dimensions seem very adequate for 4 birds. Generally it is 4 sq feet per bird for coop, 10 sq feet per bird for run.
A May chick would have been 3 1/2 months old compared to 5 1/2 month old ISA hybrids.. an EE generally matures more slowly compared to hybrids that grow quickly...My GSL's are nearly double my slower maturing Isbar at the same ages and noticeably larger than my Barnevelders. Breed makes a big difference in maturity. That could make for a significant difference in size and enough to trigger the "pack" mentality among an assertive flock. If the EE was significantly different in color, that also can flag a bird for hazing, and if it was bearded/muffed that could make a difference as well.
I actually do much better with full size birds than smaller chicks for integrating into an established flock, especially if I have some "cranky" gals...the littles can get really picked on by the big birds...and my banties were hazed horribly by some in the large fowl flock. (I actually got rid of the nastier ones as that kind of aggression is not allowed in my flock...the kind where they hold the bird down and continue to peck on her....chest bumping, short foot chases, a bit of posturing is to be expected...but never holding a bird down to pound on it or draw blood.).
While some of this can be definitely explained by flock dynamics, and introducing a single smaller bird among them, it does show your ISA's are assertive birds. Laid back or less secure birds would have been more fearful than aggressive...huddled together in one corner while the newcomer(s) were huddled in the other ..so if you choose to later bring in more birds, I would integrate in 2 or 3 of nearly same size to the flock. I highly recommend that you separate them with side by side runs with fence exposure for at least a week, two being better...and never add much smaller and never just one bird. It can take up to a month to slowly, carefully integrate new birds...I like to do it with fence exposure first, then free range exposure that has lots of room and hiding places, but everybody going back to their separate roost areas, after a week with free range together, somebody usually has broken the ice and gone to roost together and they are soon all integrated...if not, I will place the newer birds in the coop at night by my calmest birds and they wake up integrated with only a few mild tussles the next day or two.
it's not a question that your ISA's got a "taste" but rather their behavior may indicate temperaments of assertive birds who want to strongly hold the flock integrity. Most Red Sexlinks/ISA type birds are calm and laid back, but some lines can be pretty aggressive. Other birds roll with the ebb and flow of new birds coming in and you get nothing more than a couple of mild tussles and chest pumping shows. It really depends on the chemistry of your flock...never be afraid to cull or re-home any bird you feel is too aggressive....it just isn't worth the heartache of not.
LofMc
so sorry for your loss!
