As all the other fish will outgrow you tank I would keep the Dempsey and build on that.
1 sex your Dempsey
2 get 2 more Dempsey maybe a gold and a blue Dempsey but make sure you only have one male.
*Then you will have a harem.
3 buy a nice looking pleco that doesn't exceed 10-12"
4 buy a big peacock eel to replace the tyre track
5 buy a 4 lined pim catfish to replace your channel.
6 give the cats and eel hiding spots
7 watch them all grow and enjoy selling the baby's as blue gene or gold gene, maybe even grow some out and try to mix the blue and gold genes for fun.
If you see eggs you will have to divide of the others incase the eat the eggs or get injured.
Just a nice idea. Try to stay away from any fish that tops 10-12 inch.