I allways disable upnp. Router, program.
Skype just needs outgoing rules.
Treat it as a normal game (outgoing only, or UDP+TCP outgoing only). And you will recieve calls and messages because the skype protocoll lets them appear as requested packets. (You could test it with a friend).
Avoid automatic functions to create rules, especially if you know what connections are necessary!
Make sure that you look at each setting! Cookies off, etc, .... no calls from strangers, no messages from strangers etc.
Never press links from strangers. Allways think twice before pressing a link from a friend.... in case that a malware sends messages with the client of your friend.... "Is this your picture in this forum?"
It is as safe as a game. S... can happen, if you dont keep your eyes open.
Worst case scenarios.... but nothing usual.
Funny hint if you get spam:
Make a setting that "moves" you to a remote country
And dont let it run when you dont need it. The super node thingie..... just remind you.