I suspect my rat has an allergy or infection, but Im new to the rat community.

Question
When I brought home my two male rats, it only took a day or two for their personalities to develop. Barnaby Jones is more active and curious, but not too friendly (yet).  Disashi-Patrique (great names, eh?) is quieter and a bit more cautious about exploring, playing, etc. I've had them about a week now, and the entire time Disashi has been sneezing and snuffling.  No wheezing, runny eyes/nose, loss of appetite, lethargy.  His overall demeanor is the same, and he seems quite content when he's snuggled up on my shoulder. I've been told that some rats just sneeze more often than others, but I'm starting to wonder if perhaps his more laid-back, relaxed attitude could be another symptom of illness, i.e. maybe he would be just as crazy and active as his brother if he felt better.  I know that the only way to be sure is too take him to a vet, and if anything changes I'll be sure to do this.  I was just curious if there might be other explanations.  Also, how quickly can a Upper Resp. Inf. worsen, and what are the first signs that would indicate it is, indeed, getting worse?  Also, I have cats.  They are three rooms and entire floor away from the rats, but I don't know how sensitive rats are to such things. Do rats ever have caat allergies?  Lastly (sorry, this is getting longer than it was supposed to), I've had cats for seventeen years, in addition to strays I've fostered briefly here and there. I like to think that I can read an animal pretty well, and I'm pretty sure with my rats that, while they're settling in, they're still slightly stressed/confused from the move from the pet store to my home.  Is it safe to wait until the stress has gone away and their more comfortable with their new environment to see if Disashi starts being more active?  Or is it just better to get him to a vet right away?
Sorry for all the questions, any help you can offer is very deeply appreciated.

Answer
Well, you said it yourself. No wheezing, runny eyes/nose, loss of appetite or lethargy. If his coat is smooth and he's still behaving fairly normally, he may just be sneezing. If he gains another obvious symptom, then I'd worry. Otherwise, give him time. If his sneezing is excessive (more than a dozen times in a five minute period) and you begin to find droplets of what appears to be blood (it isn't, by the way, it's snot) is on the cage wall or bedding, then I'd worry.

Sorry for the delay in answer. Hope I've helped some.