Re: about training capacity
FWIW, I do know people who do both strength training and aikido and are not stiff at all. I do a little bit myself and found it helped me get less injuries, not more. I think in theory it's possible to do in such a way that it helps rather than hurting. But it's true that you have to THINK differently in aikido, and get away from the habit of tensing your muscles in certain ways. It's the mind part that's the most different, I think. And you have to be sure to train in such a way that keeps you flexible.
What I've been told by others and try to do myself when doing any kind of strength training is to focus on exercises that go through a full range of motion of my joints, ones that use my whole body including all the little supporting muscles, exercises that work on balance and proprioception and core... Things that help you be strong in a flexible and agile way, not in a stiff or resistant way.
I am not particularly big myself (by which I mean I'm 5'3" and a woman) and I am usually fine with people much bigger than me throwing me around fairly hard or pinning me. The biggest thing is moving with it, not against it, and being relaxed enough that you can feel things quickly to do that. That usually kind of neutralizes the forces. But it's also true that there are times people have you trapped and can hurt you. So communication with your partner is also part of it. Learning when to tap and when to stop your partner and tell them to slow down.