Now that I finished listen to the 346Pro selection CD vol.1 songs, gotta say that Rin's S(MILE)ING! is pretty solid once you get over the song's slowness. Sadly, I can't say the same to the other songs.
The album was decent, imo. Yes, Rin's S(Mile)ING! was the best song in the album, but I have one big problem that might seem confusing at first: its existence.
What I mean is, the lyrics can definitely be read as overcoming a sort of hardship in life and wanting to get on stage to shed all those troubles away for good. For Uzuki, this is perfect in a meta sense. Keep in mind, the poor girl didn't have an SR for a while and it caused many to feel sorry for the girl. The devs took note of this, gave Uzuki SRs, a voice, and a CD debut. Uzuki is the definition of overcoming hardships. From a struggling character to arguably the Haruka/Mirai of Cinderella Girls.
Rin, on the other hand, kinda seems like she gets everything she wants without much effort. She was the first to get a CD debut, is the poster-girl for the sub-series, and even got a voice before voicing CG idols was a thing! Because of all of this, it doesn't feel natural for Rin to sing this song because she didn't have the real life hardships that Uzuki had. I think they should've just given this to Mio, who had similar struggles to Uzuki in real life.
Btw, Nocturne was the biggest letdown ever.