if you attempt to take your shoes off when invited in, in most cases they will tell you its ok to keep them on anyway lol.
In my experience, I am always given clapki if I take my shoes off. Sometimes I am told to keep them on, but I always take them off, no matter what. Sign of respect not to bring filth from outside into the home.
Milk in tea? Yes, I sometimes add milk, but I have never seen a Polish person do it.
Another do. Always come with an empty stomach to a Polish home when you are invited. There will be plate fulls of food awaiting you. Manners dictate that you never say Nie Dziekuje when the lady of the household puts out some cake or czasto for you. Always have at least one piece, but never too many. And compliment, say Dziekuje after every meal. Not polite to burp or to fart after a good meal, or to say Oh wow! I am Full! and stretch out, unless it is family.