This is possibly the most appalling setup I have seen in my 18 months on this forum. The poster has got hold of 2 fish that grow to well over a metre (the gar and the red-tail), 2 fish that grow to half a metre (the adonis plecs) and he is proposing to keep these in a 180 gallon tank with nitrate-sensitive fish like discus and extremely nitrate-sensitive fish like stingrays. What is more, he is currently keeping them in tiny tanks, not suited to anything larger than a platy. And he thinks he can teach predators not to be predators any more, but be nice to one another
Aquaman, what you are doing is totally irresponsible. You need to sort out your current setup- if you are really going to keep that redtail it will need something more like a heated pond. The discus can be kept in a 55 gallon tank or so, but they need their own setup with no fish that can stress them out, and with low nitrates.
You have no right to introduce stingrays to any of your current fish. Stingrays are VERY specialist fish, to be kept under specialist conditions.
The zebra plecos stay small enough to live in say a 55 gallon tank, but they do require good water conditions, and the fact that they are a threatened species puts a special onus on the owner to provide them with the best conditions.
You can't just keep buying fish because you like them and expect them to adapt: fishkeeping is all about skill, research and the ability to make responsible decisions. Do you actually know that you are going to be getting a bigger tank within the next few months? If not, you should take ALL your current fish back. If you KNOW for sure that you will be getting a 180 gallon VERY shortly, you could keep the plecos and the discus, but the gar and the redtail do need to go back.
I am sorry if this comes across as harsh, but what would you think of someone who bought a threatened species of mammals, say a pair of snow leopard cubs, and then kept them in the cupboard under the stairs together with a crocodile?