A well cared for Oscar can easily get over 13" and would make that 100g look small real quick.... I recently got a Bumblebee from Rapps that I plan on raising out in a 72g bow as a solo wet pet but Im already looking for a bigger tank for it. For me if he does get a bit big I have the option to move him into a 210g. I think your best bet is just to leave him as the focus fish.