in what size tank, that would help
for tankmates in a 300 gallon eventually, pbass, datnoids, oscars fish that exceed a foot id say really just depends on the aro and its attitude
The problem is what's safe for your 4 inch arow may be food for your 4 inch arow in a couple of months depending on growth rate, lol. It's better just to wait 'til growth slows a little and they're larger.
Fed well and with good water quality your aro will easily grow 1-2" per month. Best just to be patient for a few months, then start thinking about tankmates. You could use the time to research all the species of fish you are interested in and decide which ones will go well together.