They are lovely but the red tail cat and pang will grow to be huge. I've seen both at full size, the red tail cat at the shop i work in is a murderouse git and is kept with a full grown pacu, black shark and at one point a tiger shovelnose catfish in a massive indoor pond. He killed the tiger shovel nose, the black shark( nearly 2ft long) had to be moved and was then sold and it is currently "playing" with the pacu now although pacu are generally gentle they are a huge fish and can stand up for themselves. None stand a cahnce against a red tail, thses beasts are more suited to amazonian rivers and should never be sold in shops, ours was a rescue. They grow anywhere between 3-4ft and a sheer muscle with a mouth to fit. It will kill your oscar for sure.
The irridesant shark is a member of the pang family and is a true monster much like the red tail. Growing to 4ft plus and eating anything that will fit in it's mouth. There is sadly also currently one of these at the shop, another rescue, but it is only a baby at 10 inches. These fish CANNOT be kept in normall home aquaria, it's like trying to keep an aripima (firefish native to brazil grows 5ft plus) they require a heated and filtered indoor pond of at least 5000gallons and even then they could kill one another, neither is forgiving and either one would be a formidable apponant even for a human.
My suggestion is to give them to a local aquarium society who can find appropriate homes for them and report where you got them from.
The 75g is big enough to hold your oscar ALONE it has the possibility to be a 16 inch fish with a usuall minimum (in a non stunted fish) growth of 12inches, not only that but they are chunky and usually not too keen on tank mates.
The species you own are all beautiful in their own right and in the right enviroment. But i'm sorry to say you cannot possibly house them for life, i doubt if anyone on the forums could and there are some monster keepers on this site. Look up the fish you have in the fish species index on this site, you'll see that i am not lying i also reccomend the oddballs and catfish part of the forum as well as cichlids for your oscar.
Best of luck