Re: Mixing Aikido with other martial arts
The only problem I see in training in other arts is that it will slow your progress in each.
You are right in making sure the art you cross-train in compliments Aikido well. Things that do: Certain Karate styles such as Shito Ryu and Goju Ryu. Shinden Jinen Ryu Karatedo is also rooted in Daito Ryu and could be interesting for you. Of course Judo has a long history of compatability with Aikido. I may seem bias here, but the Koroho that I am now learning and promoting blends very well with my Aikido.