These are rules:
です (formal, now) to だ (informal, now)
でした (formal, past tense) to だった (informal, past)
so the first sample you said is wrong.
お元気ですか?(it's formal, now time), the correct changing is : お元気か?(informal,; now time) the "だ" may be omitted.
Back your question..
あのロシア人はあなたの彼氏ですか? as the same changing way:
あのロシア人はあなたの彼氏か?
or あのロシア人はあなたの彼氏なのか?
When a noun at the tail of the sentence, and this sentence is a interrogative sentence, change the "だ" which at the tail of this noun to "な", and add "のか".