If one follows a school of fiqh does he have to follow it in every matter; if he does not, is he a person of bidah?
No to both questions. For example, Shafi`is in Hajj routinely follow the other Schools in the matter of non-cancellation of wudu’ upon skin contact with the other sex. However, one may not follow two different Schools inside one and the same type of worship, for example, if in the latter case a Shafi`i does not renew his wudu’ and decides to follow the Hanafi ruling in the matter, then has a nosebleed, he cannot then decide to follow the Shafi`i ruling that his wudu’ is still valid. One way or the other his wudu’ is cancelled.
And Allah knows best.