Re: Aikido and cross-training
For me I would always advise physical fitness (running, weights, whatever). But in terms of crosstraining martial arts, I would say it is great, but proceed with caution.
Training arts that complement each other well and stay out of each other's respective areas will be great for you. For example, bjj and mauy thai. A lot of students train both of these from the start. One focuses on solid striking, the other on solid grappling. In this way you are not going to get confused between the two.
A less great example would be TKD and Karate. They are very similar, but have differences that are going to frustrate you and your instructors during training. Both are striking systems, and keeping the 'uniqueness' of the systems will be hard if you just jump into both. Get a solid base in one, and the other could prove useful.
However, when it comes to grappling, I firmly believe that grappling is grappling. This is why I encourage new bjj students to try out the judo classes or wrestling classes as much as a possible too. BJJ has different strategy and philosophy about fighting, but the techniques are all the same.