If you're sure you've got rid of the scales there's no need to leave it outside. Depending what species of scale it is it may not affect your other plants anyway unless maybe they are related to the first plant. Do you think the scales came with the plant? Scale insects do occur outdoors, they're not only indoor plant pests.
I'm not aware of any health issues from them with humans or dogs.
Whether to continue fighting with the scales is a personal decision - if you have the patience to keep working at getting rid of them there's no reason why you can't save the plant.
