Probably they will do fine. Gentle mild breeds like Salmon Favorelles will get picked on by any of the production red types, in my experience.
Silkies especially are best in their own coop.
There will be individual birds who are just bullies, and need to be elsewhere; otherwise, with enough space, most birds do fine together.
Depends...on a lot of things.
How many birds (new and old) in how big a coop(in feet by feet)?
What are the ages of the birds?
New birds need to be 'homed' to the coop before free ranging them or they might not come back to coop.
Integration Basics: It's all about territory and resources(space/food/water).
Existing birds will almost always attack new ones to defend their resources.
Understanding chicken behaviors is essential to integrating new birds into your flock.
Confine new birds within sight but physically segregated from older/existing birds for several weeks, so they can see and get used to each other but not physically interact.
In adjacent runs, spread scratch grains along the dividing mesh, best if mesh is just big enough for birds to stick their head thru, so they get used to eating together. The more space, the better. Birds will peck to establish dominance, the pecked bird needs space to get away. As long as there's no copious blood drawn and/or new bird is not trapped/pinned down and beaten unmercilessly, let them work it out. Every time you interfere or remove new birds, they'll have to start the pecking order thing all over again.
Multiple feed/water stations. Dominance issues are most often carried out over sustenance, more stations lessens the frequency of that issue.
Places for the new birds to hide 'out of line of sight'(but not a dead end trap) and/or up and away from any bully birds. Roosts, pallets or boards leaned up against walls or up on concrete blocks, old chairs tables, branches, logs, stumps out in the run can really help. Lots of diversion and places to 'hide' instead of bare wide open run.
Can you put golden comets with Rhode Island reds in a coop together? Of course you can. The question is what will happen when you do. In my experience breed has very little to do with it. Many of us mix different colors, patterns, sizes, and ages, chickens with more differences between them than yours, and almost never have issues. Others can have issues even if they are the same breed. Chickens that are really different, like Silkies and their weird feathers or Polish with those strange head coverings tend to have more issues but plenty of people are able to integrate those without issues. Golden Comets and RIR's should not be that hard.
To me the important factors are ages, how much room they have inside and out, when that room is available, the quality of that room, and maybe the sex (do you have males). The personality of the individuals can have an influence too. Regardless of breed or sex, some individuals are just brutes and can make the process rough.
Mature chickens always outrank immature chickens and can be pretty vicious when enforcing those pecking order rights. If age is an issue there are techniques to handle that but which might be good for you to try will depend on the actual ages and your set-up.
One method chickens have developed when there is conflict is that the weaker runs away from the stronger or just avoids them to start with. If they can't get away then the winner doesn't know it won and may keep attacking. If they can't avoid them then they are going to spark more conflict. It's not a question of square feet per chicken, it's more can they get away or hide. Also, all the room in the world outside doesn't help when they are locked in the coop. Again there are techniques to deal with this, even in limited space, but which might apply to you will depend on your set-up and how you manage them.
If you are introducing baby chicks then sex isn't important now. If there are males in each group and they are old enough to recognize each other as rivals you will probably have issues. Again some people manage this but having a lot of room really becomes important.
You read a lot of horror stories on here about integration and we tend to make people think it is guaranteed to be a huge problem. While there are some disasters a lot of the time integration goes pretty smoothly.
The more we know about your specific conditions and set-up the more specific we can be in recommendations or suggestions but Comets to RIR's should not be that hard. Good luck!