I don't think you will get very far with a 75 gallon and caribe. I would say if you had like 4 of them at 5", that would be about the max. But those 5" fish will be 7-8" in no time, and that would be too big for a 75, IMO. They are definitely more viscous than red bellies. plus, as with all piranhas, they spook easily and go smashing into stuff, so you will want your tank in a non-traffic area. IMO a 125, or even better a tank that is 2' front-to-back would be way better.
as far as piranhas generally eating the weak ones: this is true, but id say there's a little more to it. In my experience, they will bully the weak one. as the weak one gets stressed, it will turn dark and become more timid, thus increasing the bullying. However, once this fish is removed or dies, it is quite likely they move on to the next weakest one eventually. and so on. This is not at all to say it will go on forever, or you can't keep them, but space definitely helps curb this behavior. Also, setting up the tank so that there is one major swimming area, as opposed to broken up spaces where they can establish territories, is best.
I also tend to stock with the expectation that I will lose some, but you have to be careful. The flip side is, especially with a "smaller" tank, that they do grow fast and you might have water quality issues, which could lead to more die-off than if you just had less fish to begin with. And while a larger group may quell some fighting (as opposed to only 2-3 fish), it might also work in the opposite manner if they are too cramped.
I love keeping piranhas though, so I wish you luck. they are cool. Also Caribe are cool, but WAY more aggressive than red bellies. In fact, I would suggest against putting caribe with red bellies at all.