Would you estimate Snowballs is older than Rufus?
2 is still a reasonably young cat. I find the younger the cat, the easier it is to introduce new cats. There are always exceptions to this however.
As for being able to tell if he's fixed or not.. it should be reasonably obvious if he's still got his.. "snowballs" in tact or not.
Don't give up just yet, sounds like you and your family like snowball, even though you say its been 6-7 weeks, it still could just take a while for them to get to know each other. I have 5 cats all introduced at different times, theres a gap of about 1-2 years between each. They all get on pretty well. They do scrap but its only play fighting.
Funny, the same could be said for some humans...