Yes. It is an irritant to scaleless fish, but in that level of concentration, its not going to cause the pleco irrevocable damage. The lack of oxygen in the blood, however, will.
And the liveliness of the other fish is a good indicator that its working. They are now getting only the oxygen into their blood as the chloride blocks more nitrite from entering. So, they are feeling better.
You need to find the test kit, as you will need to know if the nitrite is rising or falling. If its falling, you can do a water change to lower the amount of salt, currently at 10ppm concentration, if you dosed according to the directions above. If the nitrite is rising, you'll need to add more salt, but remember, you are already at 10 ppm for doing future calculations.
For example: if the nitrite were to rise to 2ppm, then you would really only need to dose for an ADDITIONAL 1ppm nitrite, as the first 1ppm has already been dealt with. Salt remains in the water, so you don't need to 'redose' salt on a regular basis - only when replacing water. A 25% water changes would need 25% of the original dose to keep levels steady, a 50% WC would need half of the original dose, etc.
More than likely, the nitrite will start to fall and you can begin to remove the salt from the tank via water changes. The sooner you can get the salt out, the better for the pleco.
