Certifications can only be added on a series level. We do not (yet) carry individual episode certifications.
Episode ratings often are constant. e.g. An American prime time series with episodes rated TV-14 usually won't have any TV-MA episode.
When the episodes are a mismatch of two certifications e.g. around half of the episodes are TV-PG and the other half TV-14, it is safer to add the higher certification.
The official websites, TV guides, VOD services, DVDs (season, complete series box set) and government websites can be very useful.
Unlike film ratings, TV rating can be retroactive. It's fine to add the current certification to American series that aired before the TV content rating system went into effect.
Not all types of television programs are rated.
We usually carry the original certification when the original network and a rebroadcast network have different certifications. The episodes might be cut and have a lower certification.