Names, titles and appropriateness

I have a かわいくて面白い classmate and a friend who I like a lot. She is younger than me, in her mid-twenties. I do not wish to be that formal with her, so I was trying to figure out an alternative to her five-kana name plus standard -san. I shortened it to first two syllables, adding -chan instead of -san, which resulted in マルちゃん. Does it sound strange (and especially, would マル be understood with negative connotations)? And if we were Japanese, would it be something her boyfriend might take offense at?

I think it is an idea to ask her what her nickname is in Japan and if you can call her the nickname, and you can get the right answer from her.

Oh, sorry... First of all, thank you for the reply. But I guess I did not explain well enough - none of us are Japanese, we're all learning Japanese now (still at the beginner level). I was just wondering if this would be an acceptable use in Japanese. I.e., given the situation (that I feel some affection for the younger, female friend, but we are not romantically involved), am I being too intimate or forward (or maybe condescending?) by using -chan? The descriptions of the titles that I have been able to find on the web are all rather similar, probably copied from each other, and not quite good enough to allow me to judge the attached cultural values, and the usage boundaries... She's okay with me using a nickname, I am specifically asking about the Japanese usage of マル and ちゃん.

Thanks again

About the cultural values, it must be Japanese to call someone ***chan.
But in business or formal situations, it is not a good idea to call her Maru chan.

This is a famous Maru-chan.
http://www.nippon-animation.co.jp/na/maruko/sakura/maruko/index.htm
Toyo Suisan, a leading instant noodle company, also has a character "maru chan", so give an instant noodle in the next birthday of her.
Akai Kitsune

Well considering she's a beginner too, I doubt she can "feel" Japanese yet. So go ahead and call her chan and see what happens.