It’s quite normal for learners of English to score well on one aspect of language and to get lower scores on another aspect. You could choose to reflect this in your assessment. For example, you could give a score for all or some of the following: pronunciation, vocabulary, grammar, accuracy, fluency, understanding.
By using this approach, you can give your employer information that will help them place applicants in the right job. For example, it’s important for somebody who’ll be using the phone a lot to score highly on pronunciation, fluency and understanding. An applicant for a job which involves a lot of written English will need to score more highly on grammar and accuracy.
I suggest you put together a list of questions that you can use in the interview. This can include:
questions about the person: name, age, place of origin;
questions about hobbies;
questions about previous work;
questions about the job (s)he’s applying for;
hypothetical questions, e.g. what would you do if…?
Try to include questions that cover the past, present and future to check knowledge of verb tenses. At intermediate level, they should be able to cope with most tenses. You could also use suitable pictures for the person to describe. Try to use open questions to really get the person speaking; avoid questions that can be answered by a simple yes or no.
All the best,