Are t-shirts (polo) ok to wear in churches in Italy?
Yes, no problems with t-shirts.
Yes. Your shoulders and knees need to be covered (both men and women). But how you do it is up to you.
Yes, the dress code is based on modesty rather than looking dressed up. You might have a problem with a particularly rude t-shirt but I don't think most people on this site are going to push that particular boundary.