Keeping chick age groups seperate, why? (pictures)

[flocko'seachicks]

This is my first post and I'm very disappointed it's under these circumstances. I noticed a lump on the left-side of my 3-week old bantam-cochin's face this afternoon. And tonight observed sneezing at intervals of about every 20 seconds. The breathing seemed heavy, and I'd hear a slight gurgle/weez(very slight) just before she'd clear her nasal passages again. no watering eyes, or runny nose...yet.

The chick is pooping normally, and was quite active today, we did have a little outing in the backyard 70degree weather, if that is relevant. I suspect the lump is a clogged sinus.
While searching on this forum for some answers I came across --
Quote:
this disturbed me, because some of the preventative methods for just these symptoms seems to be separating age groups in your brooder. I feel awful that I thought it acceptable to keep my 5-weekolds with my 1-weekolds. I have since separated the sickly chick, and broke up the unaffected chicks into two groups 3weeks and under and 4weeks and up.
Here are some photos of the affected chick's bump on her left side above the eye behind the beak. It looks a little pink, you'll notice the swell parts the feathers.

I've got all the chicks on medicated feed, which i understand contains antibiotics. I can encourage her to eat, but is there more I could do to cure the sniffles? I hate to take this lightly, but I'm trying hard to not over-react. What is the most likely diagnosis? What more can I do? and what about keeping chicks of varying stages of development together causes this vulnerability? Thanks to all you seasoned BYCers that can give me a clue.
I hope it hasn't spread. everything I've read about seems to spread through the whole flock, And I thought they were all vaccinated!
I've read that Tylan 50, LS-50 are best for fighting the cold... and not Teramycin. what should i pick up at the store? In other instances i've read that people just leave it to nature without medication. What is your suggestion?

 

[flocko'seachicks]

From further reading It seems possible that it is one of three things, Chronic Respratory Disease, Coryza, Infectious Bronchitis or still something else. Tetracycline or Erythromycin were on the Welp Hatchery desease chart for treatment options. Do i need to see a vetrinarian to get medication for the flock?
The bird doesn't stink or have any other symptoms... yet. and the rest of the flock is active and not showing symptoms, yet. The flock is still so young I hate for something as serious as some of the above mentioned diseases to be affecting them. I'm sanitizing everything this morning. Maybe I should set up a pen for the older ones outside... It's not that cold at night anymore. I still would love some feedback on what medicine to give, or not to give... and some possible suggestions on keeping everyone healthy.
I'll keep a close eye one everyone in the weeks ahead. Here are some more photos of the little blue Cochin, allert and awake this morning, still clearing her nasal passages every 15-20 seconds.

 

[flocko'seachicks]

This morning She's doing much better. Just lonely.... I even think the swell above her left nostril is going down. I sat and observed several minutes without any sneezing. I did however discover yesterday morning, during the deep clean, that one of the light/heat lamps was sporting a 60W bulb instead of 100W. The bulb was replaced the night before the chick got sick, by someone outside of the know. Could the resulting sniffles be caused by 24 hours without adequate heat? Is it safe to put her back with the others? I'll wait a little longer.