I am not highly ranked in aikido in the overall scheme of things, but I have also done karate, penjack silat and judo.
The only martial art I think is compatible is Judo.
Generally it is a body type, body tension issue. Karate and other similar arts build up isometric or static strength and beyond a certain point this can hinder the fluidity of aikido.
Judo builds up dynamic strength of the same type as aikido so the two are compatible.
Although I have only been doing judo for a few months, I have found it very easy to pick up due to similar principals.