word, not to hate, but i couldn't get through 30 seconds of the first one
that being said, don't quit
try to find songs that have a similar pace. that's a good way to start.
also, start off by using songs without autotune. it's easier in general to use songs where the pitch and singing and whatever doesn't change all the time. makes it sound more professional when you're all done, and like the song was originally made like that.
that's why i personally think its easiest to use artists like nas, biggie, dr dre, method man, etc as the rap, and whatever works behind them, because their voices don't shift a ton like kanye west, eminem, etc.
hope that makes sense. message me if u wanna hear my latest mashup. i'm not a pro by any means, but i know a little.