Many people unfortunately, keep inappropriate fish together. While there are exceptions to the norm, Few are those that work out in the longterm. More often as not ,as the fish grow ,,territorial and or aggression issues become more and more of a problem with some fish being injured ,or killed outright, Then the owners of these fish are faced with trying to find new homes for these adult fish which many simply don't have proper sized tanks for.
The bigger problem in my view, is that 80 to 90 percent think that they,, will be one of maybe two percent of those ,that have properly sized tanks and thus have success with keeping the fish you mentioned together with minimal problems. By properly sized tanks,, I mean tanks of 200 gal or larger.
Sure anyone can keep small fish,or innapropriate fish together. Ask them to post photos of adult fish in their tanks. I believe the photos will be few in numbers.
In closing,, I would also urge you and your boyfriend to resist the urge to feed the fish you decide on,, feeder fish purchased from fish store. These fish are often carrying all manner of bacterial diseases and or other pathogens due to the conditions of the tanks they are kept in. You have no doubt seen them in tanks containing several hundred. All it takes is one diseased fish (feeder) to infect your fish.
If you feel you want to feed a fish such as jaguar, or Oscar
,OCCASIONAL live foods,best to raise said live foods yourself.Less chance of introducing disease. Hope some of this helps you make informed decision.