If it truly is finrot, clean water will cure it.
That guppy may very well nip at the sick ones. Instead of moving all the neons to QT, you should move the guppy and pleco instead.
Finrot is caused by dirty water and cured with clean water. If the tank they are in is clean (had regular weekly PWC
and is free of ammonia, nitrite and has low nitrates) the finrot WILL NOT spread to the existing fish. However, the neons could be carrying other diseases or paracites that are contagious. Melafix won't hurt the situation a bit.
As you have a QT tank, you should ALWAYS QT new fish for three weeks or so before adding them to your main tank.
Even lively, healthy looking fish can be harboring parasites that are not visible at first take.