Is Dr. Tim's One and Only safe to put into your betta's tank in which he is already inhabiting?

I have an uncycled 3.5 gallon tank. I understand the concept of cycling, but right now, I don't have the money to buy test kits and what not. My betta has lived for 7 months in a heated, filtered tank with frequent water 30% water changes. I would like to buy Dr. Tim's One and Only or some other nitrifying bacteria stuff. How safe is it?