before i get married

What is the name of your state? What is the name of your state? calfiornia

I am going to get married. because my fiance a long time ago was incarcerated, I want to make sure about one thing: if God Forbid, something happens to him again, am I legally responsible for his child-support (from his previous marriage) and his ex-wife's debts (my fiance and his ex wife are legally divorced, but have at least 10,000 in debt)

Stop! Wait! Wait 2 years.
Do not get married until his financial situation is cleared up.
At the very least, eventually, you are going to resent the payments made toward that debt. It will diminish your standard of living. Like buying hay for a dead horse. The child support is going to be an irritation as well, especially if he buys stuff during visits.
Read all the posts in this forum and the Alimony forum. Invest a few bucks and have his background investigated. Make out a prenuptial agreement and do not co-mingle assets. No joint credit cards or car loans.
Protect yourself. Set aside the "love stuff" and pretend this is a business proposition.Do not come back here in a year to read, "I told you so".