You are right. スラスラ答える is correct. But I think スラスラと答える is more formal, and "スラスラ答える" is more casual. If you talk with your friends, you can say "スラスラ言えるよ" or "スラスラ答えられるよ" and stuff like that. But if you write a formal sentence, you should add "と". Four example... "スラスラと答えることが出来ます"
Could you understand ? I would be glad if I could help you. Thank you.