is it too soon?

Stay classy. It IS a bit too soon, that's why it feels too soon. Look, there's nothing wrong with literally just hanging out (I'm assuming you may do more than that though?), but it might show a bit of respect for your ex if you made a conscious effort not to get involved with anyone for at least a little bit longer; even a few more weeks.
On the one hand, you're single so 'technically' you can do whatever you like. But try and imagine how your ex (the guy that you really love) would feel if you get involved with his good friend, especially so soon. In my opinion, it's a straight up $hitty thing for his friend to do as well - if that's what you mean. If it's honestly just friendly hanging out, then it's not so bad. But it sounds like there's more to it than that..

A bit too soon. Still keep in touch with the good friend of your ex. If you feel later on after getting to know him, then ask for more.

I think you should be friends with other guys, but not start a relationship yet. wait a few months because at this point it is too early.

I agree with you, I think it's a bit too soon. Although your ex-boyfriend wouldn't know about it, you would feel really bad and you'll feel as if you kind of 'cheated' if you know what I mean. Maybe wait a couple of weeks until you hang out with that guy friend again (to me it sounds like your 'guy friend' has a crush on you). You can always wait, and leave some time.