This is actually the knife he has, gets to be 40" Clown Knife Fish - Chitala ornata
Your tank will be ok for about 3-4 months. Both knife fish and oscars are fast growers, someone is going to die after that.
Best bet is get that 300 gallon tank cycling now. with proper tank size your fish will be fine together. Very cool fish. I wish I had the space for a clown knife good luck and keep us updated.