Buying stock video - what's essential combining it with own footage?

[Wit]

I'm thinking of buying a few videos out of stock video. As I need some shots I can't produce around this time of year. The bought shots will be only a few seconds in my video. But the footage is not the same in frame rate. Is this a problem and can I solve it? Are there more things to consider combining different footage together? I will buy from http://videohive.net - any good?

 

 

Thanx for the help!