1) You can use any C function in C++ if you like. So if you find a function in C, then you can use it in C++
2) 'project' is sth to help you manage your files. Say your app consist of 5 C++ files, 2 lib files and some other files, Project help you to manage things such as file path, debug path, include file search path etc. So basically, project is a helper. In Borland C++ 3.1, you can omit project by using default project and default configuration( if you app is very simple).
however, In version higher than 3.1, the configurations are much complicated than 3.1.