The persistent belief that bathrooms are good for plants due to high humidity is not really accurate. Yes, we all experience the steaminess of the bathroom after we shower. However, after the door or window has been open for 30 minutes or so, the humidity has dissipated. Unless you have a very large family requiring a lot of showers throughout the day, the humidity is not appreciably better in the bathroom than elsewhere in the house. In addition, nearly all commonly used houseplants do just fine in low humidity as long as they are watered properly so they get water through their roots.
Plants need for good light is much more important than humidity, so locate your plants accordingly. Healthy plants are more resistant to plant pests and pathogens.
High humidity is a deterrent to spider mites but has little effect on other indoor plant pests. Keeping plants in an enclosed space can become a breeding ground for fungi and make it easier for any existing pests to spread to other plants. But the humidity itself is not the source of plant pests.
Fungus gnat larvae live in the top layer of soil in plants. The best antidote for them is to allow the soil to dry deeper into the pot before watering. I suspect most of your plants will do better if you allow them to dry out more in between waterings.
Mealybugs are best treated by thoroughly spraying infected plants with a solution of 5 parts water, 1 part alcohol and a squirt of liquid soap. However, you must get thorough coverage with the spray of all leaf and stem surfaces for it to be effective.
Tiny nematodes in the soil, are not usually a problem. If you are seeing a lot of them, then that is an indication that the plant is potted in non-sterile soil.
