My father has owned his CSP-150 since June, so I think I can help.QUESTION 1:
You can add new Follow the Lights (FTL) songs through Yamaha's MusicSoft website. Unfortunately, the selection is rather slim, and pricey, IMO. The good news is, any MIDI file that you add to your SmartPianist app will play using FTL
on your CSP-170. There are literally hundreds of thousands of MIDI files available online, many of them free. There's awesome MIDI files, and there's terrible MIDI files... the challenge is finding the former.
The reason your MP3 files don't work is because they are AUDIO songs, not MIDI. So WAV files, WMA files, and the like also won't work on FTL. However, the SmartPianist app can try and make chord sheet music for you and those chords will work on FTL... you just won't see the melody.QUESTION 2:
Yes, Yamaha MusicSoft is a legit website, though I personally feel it needs an overhaul: it's slow and clunky. QUESTION 3:
As I mentioned above, any MIDI file you download will use FTL on your CSP-170. Unfortunately, the SmartPianist app will not generate music that is easy to read most of the time... usually it's a big mess and has to be tweaked often. I'm sure there are music notation iOS apps which can import MIDI files and makes an easy-to-read score? I'm just not aware of any...QUESTION 4:
There's a really cool iOS app called FlowKey which shows you how to play many songs of many skill levels, and works beautifully with FTL... Did you purchase your CSP-170 recently? You can use FlowKey Premium for FREE for 3 months as part of a Yamaha promotion through December. Read about it HERE
and get your 3-month free membership HERE
(P.S. I highly recommend it if you like the FTL approach to learning music.)
While FTL is cool and fun way to get your feet wet, I'd recommend you dive into reading music as quickly as you can - this will be the best way to learn new music and how to play. A good app for this is called SimplyPiano... check it out. I'd also highly recommend the Piano Adventures line of books and its accompanying iOS app... they two things work very well together and it has become my father's favorite approach to learning music.
Hope that helps! Good luck.