Hi RatOwner! Congratulations on your new rats.
I'll do my best to answer your questions. Others here will have great insight for you too.
Sneezing: You've had your rats for almost two weeks. I'm thinking this will be past the 'new home sneezes' stage. So, my next suspicion would be either sensitivity to environment like bedding or something, or the beginnings of a upper respiratory infection (URI). Or both.
Rats have the puniest respiratory systems in the world. Stress can affect their immune system-stress like moving to a new home for instance. Anything with perfumes, dust, dirty cage fumes, or phenols in their environment will trigger inflamation and a secondary respiratory infection.
So, first of all, lets detective work the environment.
What kind of bedding do they have specifically (name brand included)?
Are they in a drafty, dusty, fumey, or moldy area?
If any of the above are the trigger, we can help eliminate that for you.
Next, lets look at the symptoms: I would bet money at the very least Houdini may have the start of standard URI. This will require a vet visit and a round of antibiotics to treat. If you need tips on finding a good vet, we'll be happy to share.
There is also a chance that your rats came to you with something more serious, and the two you sent back were also sick. Pet stores have zero effective quarantine when it comes to rodents, and the poor things are often mill bred, stressed, and with poor immune systems. If it's something more serious than a URI, it will be even more urgent that your boy go to the vet for some aggressive treatment. I'll keep my fingers crossed for you though, that it's just a URI.
Your toe wrestler is hysterical. If you've got a youngster especially, licking and nibbling are signs of affection, and wrestling is a sign of play. You may just be lucky and have an exceptionally people oriented rat.
If they are old enough to go through puberty/hormones, you might end up dealing with hormonal aggression. I'm not as familiar with all subtleties of hormonal people aggression, so I can't say that is what's going on, someone else, perhaps Lilspaz, can give a more knowledgeable answer. But my first instinct is that he's just a cheeky, playful boy.
If it does ultimately turn out to be hormonal aggression, that can most often be easily treated by having him neutered.