In CS, players more frequently use 匪(maybe short for 土匪) to refer to terrorist.
1\As I know, 土匪 is not commonly used in our daily life. Basically 土匪 is closer to bandit, which means the bad guys live in deep of mountain and live on robbing merchants who pass through mountain. To break down this word, as a adjective 土 always means vulgar, illiterate or have no fashion-sense. 匪 means bad guys(robber). Nowadays, 土匪 more commonly appears in some Chinese TV shows, whose plots refer to the robber in mountain.
2\I'm afraid we don't. The meaning of 土 confines the available scale to use this word(土匪). However, maybe we use single word 匪 more commonly to describe bad guys in video games.(especially those which contain plots about police or army)
3\To me, I will use 土匪 to describe someone really truculent, rough or rude(impolite). e.g.（someone rushes in and bothers other people）你真是个土匪。
Hope this helps clarify your question.