Sounds like you bought some sick birds.
What ever you do, DO NOT let them come even near to contact with your current flock. Wash your hands between your current birds and everything that can possibly come in contact.
The bugs are probably lice/mites. You'll need to find a poultry dust powder at the feed store to kill them. May take a few treatments. Then the straw sound, that sounds like a respiratory infection. You'll have to look up that topic on the board as I don't have much to add besides they are usually viral so can be very contagious and can't be beaten with antibiotics because they are not bacterial. They usually have to run their course.
On size... it really depends on strain. In Europe, there is a bantam and a standard size silkie, here in the US, ours are in a rage all between the standard and bantam size of Europe. So if you just got them, I'd worry about their paracite and health fire before worrying about size.