I have already the Alesis io2 Express USB Audio Interface. I used it to connect my microphone and MIDI keyboard (the interface has midi in/out) to my computer. Now I want to buy a digital piano, which has only an USB port to connect it with the computer. Will both devices: the interface with the microphone, and the digital piano via USB work together /connected parallely?
Yes. There can be problems connecting two audio interfaces to the same computer. But the piano isn't an audio interface. It uses USB as a MIDI connection - you're recording the keystrokes, not the sound.