what are the chances my 1 1/2 year old roo tries to kill my 9 wk old roo?
It just depends on a lot of factors. If the chick is still very chick like, the odds are better. If the mature rooster is a lot bigger than the new rooster, that helps too. But by week 14-16, sometimes even earlier, that younger rooster is going to get interested in the ladies. IF the older rooster sees that as a threat, then it can get ugly, as to the death.
If you are going to try two roosters, do it ASAP BUT not before you have a way to separate them set up. If you have to separate them, you have to do it right now, you won't have time.
Mrs K