After reading a bit it seems most all things in chickens is treatable but not curable so even without signs of anything can they spread it to the rest of the flock?
Yes, that's true.
Many of the avian diseases can lay dormant, not showing symptoms or causing problems, unless stress becomes a variable. These can be passed to other birds, or other birds may already be carrying but also not show symptoms.
That's why a seemingly healthy looking bird may start showing symptoms when brought to a new place.
'Stress' can be caused by many things, like introduction of new birds.....
.......or removal of birds which changes the pecking order, a predator attack, loss of nutrition, weather extremes, change in housing, etc.
I wouldn't treat unless things get extreme and you know
what to treat for.
Check them over for external parasites and just observe their behavior and condition closely.
It's going to take them some time to settle in and get comfortable, just give good feed and water and see how thing go.
Just my 2 cents(sense?).