. I guess the more flexible you are the less likely you are to injure yourself in Aikido. Am I correct?
Yup, you are correct (at least, I believe that will help me with my aikido).
That and cardio; nothing worse than when the mind, spirit and body are willing and able to keep taking ukemi from a sensei or fellow student and what drags you down is your aerobic capacity.
Answering your original question...
"When you think about somebody with large amount of muscle mass starting aikido, is there any advice you would like to give?"
Use what you have, you are after all, you. And like noted above, stretch.