if a lawyer on here offered to take my case and I declined, can my ex-husband now hire them against me?

This is a question of fact, exactly how far things went with the lawyer. But, based on what you say, you refused the attorney's offer and never retained the lawyer. I'm guessing that no attorney-client relationship was formed between you and the lawyer. In that light, yes, your ex-husband can do that. Which means, you need to hire your own attorney, an experienced family law attorney to assist you in presenting your claims and defenses. And, your own attorney can review the actual facts of your question, and if appropriate, file to have that other attorney removed from the case.