Re: Aikido against strong people...
There are basic kihon exercises that I had to learn with a man who could bench press four hundred pounds. Katatedori, morotedori, kubishime and ushiro ryotekubidori all stand out for me. Tai no tenkan and kokyudoza are things I like to practice with someone giving me power. Let uke get a solid grip and give enough that if you aren't moving with good alignment nothing much happens. I like working these alignments because this translates into good alignment for everyday life, and higher functional strength.
However, timing and placement are a separate practice and with kuzushi Uke is less able to bring strength to bear. We're all built in such a way that in some directions we have little strength or a more easily compromised balance.
When strength messes up practice is when uke knows I am, for example, supposed to lift up. Uke bears down. I am "supposed" to clash and resist their direct force. When uke pushes down, sink or lead out. Don't practice beating your head against a brick wall. If uke's hands are stiff, then you can probably punch him in the face faster than he can release tension and respond. Otherwise, it is common for beginners to make kaitenage easy by resisting iriminage and visa versa, or twisting into kotegaeshi to avoid nikyo.
There is a lot of fun to be had with strong uke, but strong beginners are often brittle. Resisting also sets up bad jiyuwaza ukemi as all the exactly wrong reflexes were trained. So, when I train against power I move very slowly so uke does not get damaged, but I also am not using speed to cover up a hole in my own structure.