Whoa, there. One step at a time.
Short answer - yes you can keep an arowana with rays. How many and which species is best for beginners is subjective. In my opinion, you could keep 2 or 3 smaller rays (p. motoro is the gold standard starter fish) in a 180.
Long answer - Start reading.
This is a good book to start with. Spend some time on the net before blowing a whole bunch of money.