You need to be careful introducing new birds to a flock.
(PLEASE be sure to do a quarantine period with the new birds before they are allowed anywhere near the other flock!!!)
Occasionally it will go without any hitches but you really don't want to assume that all will go well!
The consequences are really horrid and pretty dire (death or mutilation) if it doesn't go well and it can happen fast. Chicken03 I would put the new chickens in a cage or enclosure next to the others for about a week. Then what I like to do is release them all into an area that isn't 'owned' by any group. Then I hang out with them for as long as it takes, on the initial intro sometimes not long at all. As soon as things start getting too intense I put them back into their separate areas (a helper can be a good thing depending on how well your chickens do with being picked up or shooed into/out of areas) I do the supervised 'social time' as often as I can until they seem to have ironed out the most serious of the pecking order issues. I don't allow them to gang up on anyone, a little sparring and pecking, the occasional feather yanked is pretty normal. But if it gets too intense I break things up pretty quick. This has worked very well for me this far... Make sure the birds have things to hide behind or on top of if things get tough. I find that having some hay bales or higher roosts can really help and plenty of space so they can get away from each other if they need to. Also make sure that you have feeders and waterers in a few different places so that everyone can get to food and water without having to go past that dominant hen/s to get there. Sometimes it is just easier to get a new flock altogether rather than hassle with the introductions. It also depends on the breeds you are dealing with, some are a lot more tolerant and less aggressive and others will not tolerate newcomers at all and there WILL be bloodshed.
Chickeneer, yes your remaining bird will not like not being in a 'flock' of some kind. In the wild they are much more vunerable on their own. HOWEVER, I would never put a much smaller bird with the older one. THat is a recipe for disaster unless the larger bird is the mother. If you want to get those two together then you will have to wait until they are nearer in size and then try. If you keep them next to each other but separated, the introduction will usually go a lot smoother when the time comes. Of course there is a lot more pressure on the relationship because there are no other birds to diffuse any differences if things get heated... and it doesn't sound like you have much space ... good luck figuring things out.