Hi Leslie,
Don't start by throwing plants out. Just make sure you are taking care of your plants in the right way. Helthy plants hardly ever get infected by insects. My banana tree & ficus grow happily side-by-side in my office. The banana tree gets mites every few months, and looses a lot of foliage (Whenever the heating kicks in the air dries out, and the banana plant suffers). However, te ficus has not been affected. Yet at home, the same species of ficus gets mites all the time, as the soil is too dry, it sits next to sliding doors => drafty.
Cacti most definately can get the scale too. Use the same solution as before, and use cotton swaps to treat them, one insect at a time. Are your cacti happy ad healty, nicely growing? Note: Cacti do not drown easily, when taken care of properly: Water only when the roots are 'active (In dry spells the roots dry out; Watering them then will result in massive root die-back. Only when temperatures rise, and air-humidity is brought up e.g., by misting every other day of a week or two, will the roots fill up with moisture and be ready to receive watering. Make sure the water can drain away, and do not water untill the soil has dried out considerably again. Yet, letting the soil become dus again is most of the time too dry, also, only water when it is warm; cold weather & wet roots don't go well together for most succulents).
The insecticide should work. However, again, the little buggers will be back if you do not figure out what is wrong in the plants' life; Happy plants do not get pests all that easily. It is mainly stressed plants where pests can become a life-threatening plague.