I think most people can have a successful relationship if they care enough about each other to work on it. Too many people walk out at the first sign of trouble. My philosophy has always been if you break up once, you will always continue to do that, so a couple that breaks up instead of solving their problem will never make it as far as a successful relationship. That's a life lesson, not an astrology one because we can't leave our common sense at the door.
Scorpio and Sag are not "complete opposites," I don't know who told you that. There are 12 zodiac signs, so Taurus is opposite Scorpio and Gemini is opposite Sagittarius. Actually, Scorpio and Sag moons are zodiac signs next to each other, so close signs like that usually understand each other; this would be about emotions since they are your moon signs.
Since you both have Leo suns, I don't usually see many successful relationships with people of the same sun sign. They are usually too much alike. It's possible, but it's not a passionate match. I know a few couples of the same sign, but they act more like friends.
I think you are telling me both of you have your moon in the 8th house? You are more emotional than he is. Both being Leo, you both want a lot of attention and to be the King or Queen, so you are both high maintenance people. It's hard for two people like that to be together for the long run. I don't think it's a terrible combination, but that you are too much the same, and even the moon signs in zodiac signs next to each other isn't really a good romantic match. I would say give it a try; you may be surprised.
Also, interpreting what a planet in a zodiac sign is in is not anywhere near a whole astrological horoscope. So I answer the questions as best I can, but most of the people who ask me for this little bit of what a chart would be are getting a small window of what is really their natal birth chart. I have to spend a half hour on some of these questions as part of a group site I write on and am used to casting whole charts and getting paid for my time. So this feature leads people to believe they are getting more than they are, and my answers are not complete since I don't have the right information, just little bits.