Hello, I think I can answer your questions.
1 - No, diffraction and interference are different. In the simplest terms and most convenient definitions, diffraction is light bending around a corner, or slit if you will. Interference is the constructive and destructive sums of wave amplitudes. Interference happens as a result of diffraction as the wave now has different point sources from a change in direction of light paths.
2 - Actually, it is not necessary for the slit to be comparable. Diffraction will always occur if there is a corner light can bend around. When you learn about it in the classroom, what they mean by the slit being comparable or smaller is the case that it will generate point sources that will make the interference pattern look like a single radiant source, like when you drop a pebble in a pond. The slit must be smaller (or comparable to the wavelength) so that the wave must "squeeze" through the slit. Slits larger than the wavelength need not to "squeeze" as they can fit through the slit just fine. However, there is light hitting the corners of the large slit thus having some light changing direction. The result is you see the interference pattern downstream of the slit rather than right at the slit.
Keep in mind diffraction is also dependent on the incident wave. Usually at this level the incident wave is always a plane wave of single wavelength.
Hope the squeeze analogy was enough for a qualitative answer.
