I'm probably the least qulified person on this site to answer so I will anyways
It's all up to you in the end my friend. We've touched on it in a few other threads here.
Mixing your martial arts will have pros and cons.
Some people will try 2 or 3 at the same time sacrificing "mastery" for exposure to different styles while some people will devote their whole time to aikido.
You won't get a right or wrong answer. I'm guessing your looking for a what belt should I reach before trying something new. Green belt *might* be a good guess (as such its mine) but your going to get a bunch of opinions.
Taking on more than one martial art at a time is tricky because you're having to learn twice as much. Different names, different languages, different stances etc.. etc..
It's all personal choice was the basic answer to simular questions in another thread.