The bookshelf tank would be decent for a pair of Lamp. brevis IMO since they're not very outgoing, I never had a pair that took up more than half a 10 gallon tank. Other than that it's not really suitable for any other cichlids.
I've had the 6.6 gallon bookshelf tank. This was my tank as a Nano Natives Tank.
With the high bioload I had to upgrade the filter as the one that comes with the tank is not sufficient for anything more than a betta.
The only non-natives were the school of young Rasbora dorsiocellata I planned on replacing with Leptolucania ommata but I never got around to it.
In the tank I had a few Lucania goodei, Etheostoma fusiforme and a pair of Elassoma sp. cf. okeefenokee.
You could also do a South East Asian version with Dario dario or Dario hysignon with a school or Boraras sp. Badis and Dario species are very similar in looks to cichlids, so much so that they were almost considered cichlids.