Is the term "medior" to express "medium level expertise" wrong?

A quick check has shown that 'medior' does not appear in any reputable dictionaries, not even in ones of slang or jargon. We can conclude that it is not a widely accepted word - not a 'real' English word in any normal sense. That does not stop you using it if you think it will be understood, but most people won't be familiar. Some Latin speakers will probably understand it.

The normal word for "between junior and senior" is "intermediate".

The word medior is used in Dutch to express exactly what you mentioned. According to Encyclo.nl "Medior is used in various contexts as a name for a member of a particular middle group. For example, in job vacancies medior stands for 'member of the middle management', between the seniors (staff members) and juniors (starting employees)."