Quarantine any new plants in a separate location, if at all possible. Some fungal pathogens and some insect pests can be explosively disastrous. Either can start being visibly a problem without warning, weeks after the pathogen/pest gets introduced.
I used to advocate indiscriminate spraying of new plants with 1% hydrogen peroxide. However, any plant that has, previously, been sprayed with potassium stearate (insecticidal soap) or oil-based sprays (such as neem) is going to get badly scorched and burned. It would seem that the wax layer on the leaves (rather like the epidermis layer on our skin) is compromised by these chemicals.
I still recommend frequent spraying with water, and pay special attention to leaf undersides, during the quarantine period. Watering in a colloidal suspension of diatomaceous earth (3 Tbsp in 5-gallon bucket of dechlorinated water), once ever week, will eliminate fungus gnats, springtails, thrip pupae, etc. -- most soil-borne pests. As diatomaceous earth degrades and dissolves, it releases water-soluble silicates that some plants use to improve their defenses against diseases -- i don't know if peppers benefit from this.
Sticky strips for whitefly are an asset.
I also sometimes use a preventive spray of 50% Listerine mouthwash, diluted with 50% water -- the active principles are menthol and thymol, and they are effective fungicides... a purist could try applying sage oil iin some fashion, i suppose. It's the same two chemicals in each source.
So far, this hasn't had the hideous side effects i've occasionally met with the peroxide spray.
It also works well outdoors, on roses, to treat powdery mildew and sometimes anthracnose.
Note that this will also kill off any beneficials introduced (ie.: predatory mites).
EDIT: Error here: should have said 50% Listerine, 50% water. Not "1% Listerine" as originally posted.