I think I'm leaning towards getting one or two pullets.
1. Would the two newbies not gang up on the older hen?
2. If I got just one wouldn't both be more inclined to make friends rather than fight and stay on their own?
3. Or maybe get her some fertile eggs to hatch?
1. Could go either way. The existing bird(s) usually attack the newcomers to protect their territory and resources(feed/water/space). Size and numbers don't always help.
2. Not necessarily.
3. Is she broody?
I really wish chickens were all happy-go-lucky relaxed friendly creatures, things would be so much simpler!
Romance meets Reality.
The respiratory issue is a serious consideration....your existing hen is very likely carrying that illness, even tho she may not have any obvious symptoms. It's a risk you'll have to weigh if bringing in other birds. The newbies may also have pests and diseases that they could give to your existing hen. Integration is stressful, stress can bring latent disease to the forefront very quickly.
Another thing to keep in mind is that integration is best done with 'extra' and separate space...do you have a way to set that up?
Consider biological/medical quarantine:
BYC Medical Quarantine Article
BYC 'quarantine' search
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.
This used to be a better search, new format has reduced it's efficacy, but still:
Read up on integration..... BYC advanced search>titles only>
integration
This is good place to start reading, BUT some info is outdated IMO:
http://www.backyardchickens.com/a/adding-to-your-flock