Multiple aros is possible. Aros can be very territorial, so that is why you see solo aro tanks often. Search around the site, you will see it can be done. Make sure you get them big enough that the rays don't turn them into a snack. I would introduce the aros at the same time to improve long term success without constant fighting within the aros. It's certainly not a science and individual personalities will play the biggest factor. South American aros might give you the best odds. A guy close to where I live had two 28inch black aros with a dozen or so rays in a 800gal. His mix worked fine, never had a problem. I would ONLY try it if you have another BIG tank to separate or somebody with tank space to house the picked on in case of a sudden crisis..If you don't have these options, one full grown 3ft+ aro is nice and will fill out the top of the 600 just fine imo