Scale is ubiquitous in nature, and exist everywhere, no matter what plant you have when you bring it indoors there will be no predators to eat them, so don't worry about good/bad plants, bugs like them all! The life cycle is something like 2 weeks , so spraying at 2 week intervals, 3 times with neem oil should take care of most of them.

The hard outside is to protect them from desiccation and predators. the alcohol dries them out and the neem (or any oil) smothers them. The neem oil has the added benefit of disrupting the reproduction cycle, so used 2-3x at 2 week intervals has great success. I started when I brought them in, and once after for most, and 3x for a few more susceptible ones.

Most scale are introduced by birds; but once you have them, they multiply like crazy. We treat our Meyer trees with systemic Imidacloprid which does a pretty good job of controlling them, plus it doesn't kill the beneficial wasps that also really help to control them. For my garden trees, I prefer to use hand removal and smash them; or rub them off with a Q-tip and alcohol.