Re: Aikido against strong people...
Applying more force seems like it would always be the wrong answer in training. If you are applying the technique, it will work. If it isn't work, you aren't applying it (i.e., you aren't doing it correctly). Doing the same thing harder isn't going to make it more successful, but it could result in injury to yourself or someone else.
"But what if I'm attacked for real?" Is that something that happens to you on a regular basis, then -- or that you know will happen to you at some already-determined point in the future? No? Then it's in your best interest to train as best you can to apply the technique correctly. If you don't, and you ARE attacked for real, simply applying more force may very well not get the job done -- and that's the point. Sometimes I tell newbies that aikido assumes that there is always someone bigger and stronger than you. Safer to assume that your "for real" attacker is that bigger, stronger person, and that you will not defeat them with size and strength. So, learn aikido.
(and if you DO have immediate self-defense needs, then by all means keep up your aikido training, but don't try to press your aikido into service for self-defense yet. Just as if you were a novice pilot who had just had your first lesson flying a Cessna, if you had to get around the world, would you try to use your newfound skills to do it? No, you'd go buy a plane ticket. Use the same common sense here.)