That's the reason I originally got my 1st Royal (L191 also)-was to go with an Oscar, they're a good match. The L191's get about 9-10" max according to PlanetCatfish. But Royals grow so dang slow. Mine was 3-3 1/2" when I got him almost 2 yrs ago, he's 6" now. By the time he was big enough that my Oscar wouldn't have him for lunch I'd given up on putting him there. I made him my Plec Tank- but there's others in there, too.
The white band on their tail means they're a juvie, they'll lose that as they get older. A 75 g would be good for a few yrs but I do think you'll want to upgrade to a bigger tank. Oscars get well over a foot after several years of good care and Royals have real wide thick heads and chunky bodies.