No, they are different much.

SURE YOU CANNOT. There are some similiar words but for you it will be difficult to understand them. Native russians cannot understand serbs. As for me I can understand about 50% of words but they sounds deformed/distorted. But I know Belarussian and Ukrainian. And that's a BIG error to consider that since these languages are slavic you can speak all of them no matter what you have learned. The difference between them is much more than between English, US English and Australian English. But of course it is much easier to learn another slavic language when you already know one.

I think big YES, you can communicate with serbs.. Both languages are Slavic, and from the same root.. I have learnt russian for a while, and it was very helpfull for me to contact with serbs, the other thing is that you can either communicate with Bulgarians and also Czec and Poloni. you can test what I said usin the Google translator to see how close are those languages!!!

It's almost impossible. People from Serbia or Croatia need none the less 1 year for adaptation to Russian language...

No, they are very different. Even when I first heard a song in Serbian, I couldn't understand anything. Yes, they are a litte similar though, but if people whos native language is Russian almost can't understand Serbian, I don't think foreigner will be able. Good luck on learning russian anyway! :)