I know that both the conditional sentences below are acceptable -

If I were an animal, I would be a horse/If I was an animal, I would be a horse -

but what about this: 'It would be unwise for a person who were only a junior in the company to criticise the boss'?

You could say, 'Were a person only...' or 'Were a junior to criticise...' but using 'were' after the relative pronoun 'who' sounds strange to me.

Regards Jonathan Jackson