My other thought for you would be insurance. If you are at a gym, you’re probably covered. If you’re going to train at someone’s home or apartment complex, you should get insurance. In that case, I think you’d be very wise to get a personal training certification.
A thing to keep in mind at all times, whether you’re in group exercise or personal training or swimming lessons, or aerobic kite-flying, is scope of practice. Teach what you’ve been trained to teach, what you’ve prepared for, what you’ve thought about.