They are RTM for sure. Either way, your tank size works for them and you can mix in other species but I’d keep an eye on them as they grow. GTs aren’t tough fish when compared to RTM’s but can work together. Also, Oscar’s are much more likely to be bullied. I wouldn’t do them with a Parachromis. Also, Oscar’s can thrive in almost any tap water regardless of the type of water they are found in from the wild. Either way, don’t put them in there.