I started iai at around the same time as aikido as it's a part of our aikido syllabus to know some. I'd consider the iaido to be maybe 5-10% of what I learned over the last ten years. So a few yearsa go I decided to 'learn iaido properly' whatever that means, trouble was there wasn't any iaido here so I did kendo instead (lots of fun). I've recently begun to learn MJER in what I'd consider to be a proper way.
Interestingly, the instructor at the MJER seminar I was at last weeknd mentioned that learning two or three arts at the same time is a bad idea as it gets confusing and muddled. I think I agree with him, you should probably have a good foundation in one art before you start another (if you intend to take it seriously), for me at least its taken ten years before I've been happy with the foundation I have in aikido to start looking seriously at other arts. By which I mean that visiting other places and cross training occasionally or going to seminars isn't the same thing as serious study for at least a few years on a regular basis with a specific teacher.
It was interesting to me to see that it was more my kendo habits that crept into the MJER last weekend than my aikido ones, reckon I've got the aikido pretty well compartmentalised, but kendo is still new to me. I suspect it won't be long before I pick one to keep going with over the other (either kendo or iaido), I reckon it'll be kendo that loses out. Maybe in another 10 years I'll come back to it!
WRT to fumikomi, yup, it's really good for any and everything involving irimi in aikido because you're used to driving forwards with power and intent, exactly what irimi needs to be, a deep entry into your opponents defence. The stamp is optional (as it is in kendo AFAIK) it's the intent and power from the centre that counts. Also I've found that kendo has helped me to understand the line of attack more. It's often heard in aikido that you should get off the line of attack. Kendo really helps you understand how to dominate and redirect things along that line, be it moving yourself off it or moving your opponent/uke off of it.
Haven't been able to go to kendo for months for work reasons
really miss it and am looking forward to getting back to it in January. Be kind to me when I do Jo