I echo what Tor-Eriik says,let them get established before you go putting tank mates in.
Dorado,Mmmm ! there are much better choices,these fish are quite aggressive towards many other midwater tank mates and like already mentioned grow huge not to mention very avtive,a much bigger tank than yours would be needed in the future.
Hystrix being one of the smaller more managable species can be kept in a tank like yours for life,tank mates for me would include Geophagus,Satanoperca,Hero's sp,disc characins are good too,Metynnis sp,Myleus sp and mylossoma,and if you like your predatory fish there's south American spotted Gar Boloungerella maculata(spelling) B.latistriga.All these fish make good tank mates for smaller rays but of course they all need to be added at sizes that are compatible,the Gars mentioned do make large fish in the wild but over a 40 year period of keeping oddball fish i have never seen any of the these Gars mentioned grow larger than 12" or so in captivity.
This would make for a nice peaceful community,very Amazonian! Many of the fish mentioned are often kept by Ray enthusiasts only to be molested and killed by the ray at a later date,because Hystrix and Scobina remain quite small these choices should cohabit for many years to come.
Good luck!