Your birds so far come from breeds that are known to be calm, and generally friendly birds. ISA browns are anything but that. They are very aggressive with other birds, and I've never seen a healthy ISA brown back down from a fight (which most of the times, they win). In addition, most of your birds are bantams; that adds an extra level of concern.
Interesting. It sounds like you have had unpleasant experiences with ISA Browns. I have no doubt you saw what you saw. But I'll offer a different perspective.
ISA Browns are commercial laying hybrids. They are typically kept in commercial flocks in relatively close quarters with a large number of other ISA Browns. If they were all as mean and vicious as what you saw they would be unsuitable for a commercial flock.
My perspective is that each chicken is an individual with its own personality. Each flock has its own dynamics. If you read enough stories on here you will find stories where Silkies, Wyandottes, ISA Browns, Marans, and all other breeds can be either terrors or are the sweetest nicest chicken ever. I do think the flock they came from makes a difference. Inheritance is a factor. From limited experience with different breeds and from reading stories on here I do not put much if any faith in the breed indicating the personality of any individual.
Hello! My flock of ten is currently in a 180sqft run (20sqft covered by second coop).
A 20 square feet coop (about 4' x 5') for 10 chickens? Granted some of them are bantams so they might take less room. By the magic numbers many on this forum believe in your coop is already overcrowded but your run has plenty of room for more.
In England your climate is probably pretty mild. They will probably have access to the run most days so they should not be locked in the coop only by weather. It looks like you joined the forum this year so I don't know what experience you have keeping chickens in winter where you are. You may be fine this winter or you may be overcrowded. I would not recommend adding any chickens until you have gone through a winter.
With that coop I'd be very reluctant to add any chickens without providing more room even if you make it through the winter OK. Integration usually takes more room than they need to live together after they are integrated.
In any case, I agree with the others. I would avoid adding any single chicken. Sometimes you may have no choice, say you hatch a single chick or whatever. My minimum number if I have a choice is three. That's so if one dies they still have a buddy. Adding a single chicken can be a lot harder and more dangerous.