Is this a terrible fish combo?

[Whatigot]

sure.
though certainly I've seen successful "cramped" tanks before...
again, this is assuming diligence with weekly maintenance and adding all fish of the appropriate size at the same time, but if you look at the first post I made, I had kept a 12 gallon aquapod for a year and it was even more "cramped".

Your aquascape will be key, as long as each female has AT LEAST one separate place to call home (like an overhanging rock, a cave or some good macro growth) you will really enjoy watching the larger male "tend" to them both. doubt you would see any significant aggression unless one of them actually spawned, and even then, once a harem is formed, they tend to tolerate one another very readily and have even been reported to share in the husbandry new fry.
"cramped" yes, impossible?
no.