Yes, there are LOTS. I would need more info on your stocking.
If a BKG is a black knife, I wouldn't even bother with the tetras lol. Maybe a big school of silver dollars, maybe.
My first thought for cichilids was Boulengerochromis microlepis, but your tank is way to small to keep this fish much less with anything else. You are also keeping soft water species that like lots of cover, so you can throw any rift lake species out the window.
There are african river and american(north and south) cichlids that fit the bill.
-At 6" Haplochromis burtoni may work, but they need to be kept 1M:5F but may give your knife and clowns a run for their money.
-At 18" Oreochromis mossambicus may work, but I know little about them, (ill leave that to you
)
-Anything in the Tilapia genus, once again, agression is a concern
-Geophagus spp, are big enough and on the peaceful side of things; just make sure that the species you go with will be big enough as an adult. G. brokopondo would be a poor choice.
-Satanoperca daemon/lilith peaceful side and large enough
-Vieja synspilum 17" do your homework
-Hypselecara temporalis/coryphaenoides
You get the idea, just do your homework on whichever you decide to go with.
http
/www.davesfish.com/ often has many of these fish, I have dealt with him several times and I highly recommend.