My ex and my frenemy

Ok. My turn to ask a question.

I broke up with my girlfriend a few weeks ago due to some irresolvable differences. I met my ex at work and we dated for a little over a year. Before my Ex I was into another girl (also from work.) I liked her but she never gave me the time of day. Well she and I had a big falling out over it and after that we kind of became Frenemies. I still talk to her, make jokes with, and hang out with her at work but we also argue, fight, and make rude comments to each other like we were married lol. She is the only other girl I have ever really liked. I admit that its not a good idea cause I’ve been bitten by her before. The problem is now my EX and this girl have started hanging out together. Would it be wrong of me to pursue this chick even though my EX considers her a Bestfriend? Is it a good idea for me to pursue her at all considering our past? The last thing I want to do is hurt my EX again because I still consider her a good friend, but I don’t want to hurt myself either. Being reject could hurt but not knowing could hurt worse.

Thanks, Fireson13