On Salt: 1/4 dosage should not hurt the pleco (I did this without any bad effects).
On Tetracycline: if you can get it, it is a good choice. And I'm not sure that the concerns about the good bacteria are warranted: (1) the TC package I have states that the filtering will not be harmed or something to this effect and (2) I did run full dosage TC treatment on a set up tank and measured ammonia/nitrates twice a day -- no spike seen.
Good luck.