To elaborate on Ogerton3000's comment.
If you want a low maintenance system (because of time constraints) only a few small fish, should be your aim, and few, if any IMO, would include cichlids.
Fish that survive low quality water might be best, fish such as small live bearers, small bichirs, or other swamp fish like certain anabantids that can live in less than perfect conditions.
These type ish won't die or get diseased if you skip a water change, or change your decor, or tank mates on there own while you are not looking.
As any cichlid grows it needs more care, not less.
Where a 3" cichlid might require 1 water change per week, as that cichlid grows it needs more, and will need frequent regular filter cleaning to maintain proper health.
Severums and angels are perfect examples of these more demanding species.
They require very pristine water, or as they grow and mature, they tend to get scarred up with "hole in the head disease", due to stress brought on by high nitrates, and being held in crowded conditions that don't get enough frequent water changes, and get stressed by cramped space with too many fish).
Pleco's can also put stress on roundish flat cichlid species because in small spaces, they, ( being nocturnal) will suck the slim coat off fish like severums at night while they rest, many are opportunistic carnivores, not simple algae eaters.
By your comment about letting the tank sit only a week before adding fish, as your cycle tells me the tank was not cycled at all.
It usually takes a tank 6 - 8 weeks to cycle, all that time, it must be fed a source of ammonia, to build up the population of beneficial bacteria enough to keep it cycled for the large number of fish you added.
I consider a 75 too small at tank if there are more than just a couple severums.
Many people who own 3" severums may say they are doing fine in a such limited size tank after 6 months or a year, but a severum is a long lived (10 years) large growth (10"+) animal that unless treated properly suffers in such a tank, as is obvious by scarred up specimens turned into LFSs that have been mistreated.
Below a Pleco, chomping down on fish flesh.

As Ogerton3000 mentioned, you should "not" be adding more fish to an already compromised system, until some sort of equilibrium is maintained (if at all).