I have a couple oscars that are about 10" inches.
I am looking to get an arowana that will be around 12". Do you think there will be an issue
The reason I said "case-by-case" is because I didn't work out for me.
I tried to add a 7"-8" oscar in a 125G with 2 silvers and green. In less than 2 minutes, I could tell the answer. All arowanas were 14"+
Having said that, if your tank is bigger than 125G, there might have a different result.
If you have a choice, why don't you try other tankmates that will have a higher chance to work. ie Tiger / FF / Stingray / Endlicheri / P-bass BUT except for giant gourami and Pig-nose-turtle!
For giant gourami, I strongly recommend you to start with a small one (4"-5" in size) and grow it gradually in a comm. tank.
For PNT as well, when they grow bigger, they might attack (or bite) arowana(s) for no reason at all!!