Re: Yoshinkan and "aiki"
I would not have thought that there would be much of a problem incorporating another method into your practice, after all it is how these (all) MAs evolve. The mind body connection and pathways within the body are critical to internal martial arts, if you don't spend time developing them then you will always be missing something. In CMAs this is the Qigong and jibengong (basic) practices which should be continued ad infinitum.
Though i have never met or trained with Dan, Mark or anyone associated with than group if i did and found they had something that was more efficient or effective than my own training i would certainly find it a useful addition to my own training. I wouldn't really see the need to discontinue what i was currently doing, just have to work harder.
In the past many others have done the same eg i think Mas Oyama founder of Kyokushin Karate studied with Yoshida Kotaro a long time student of Sokaku Takeda.