Your Red Tail will probably die if put in salt. I don't think they tolerate even a little bit. I think there was a member who lost one within minutes of putting theirs in a salt solution. It may have been a Bala, but they would probably be the same.
"I used a salt bath on my bala and she was dead within the hour! I am not too sure if I would ever do that again!" quote came from from Jan
I might add while it probably wouldn't kill him the Bristlenose wouldn't be thrilled about it either...
Because the Mollies have been bred for so long in captivity without salt, they should be perfectly fine without it.