First of all, giving an opponent troops is a violation of the rules. If you do this, just be aware you are playing by house rules. As for keeping the number of cards secret, I find this to be a matter of taste as to whether you allow players to keep the number they have a secret. I usually like to know so I try to keep track in my head anyway.
Also, it is a rule that once you have 5 or more, you must trade in at least one set.