Re: Mixing Aikido with other martial arts
I agree on the BJJ. Judo is usually the first on my "self defence" list for people given that it has more standup - but I found that my aikido trained body movement often got in the way of judo body movement and I've seen it happen the other way around as well - so maybe they don't go together as easily.
I disagree that doing two arts means you won't progress in either as quickly. Provided you are not sacrifcing a training spot in art A to train Art B this should not be the case (i.e. you are expanding your total training hours rather than putting more variety into your existing ones). Particularly if the arts you choose are complimentary, as BJJ/Aikido are in the ways Kevin outlined. Indeed when I was training both I found that the insights of one often carried over into the other and made my practice better.