Hillari..... Yes, they aren't exactly common or easy to find...... Virtually impossible up till about a year or so ago.
Seem very slow growing for Dorado.
I have a group I had to split up after the alpha killed one and was on its way to offing another. I tossed it in with the Brassies. Apparently they are ok with him as he's been with them close to a year now. My other 4 reside in a 300 with a lone tail-less pink tailed Chalceus. They killed my red tailed cudas. For the most part the group is usually flawless, but do have lil squabbles every now and then which I'd consider perfectly normal for these guys. I'd say its a safe bet to say smaller, more delicate fish will be killed. May take a while but eventually they weed out tankmates not able to hold thier own as these silver Dorado can join forces and the shoal can be quite menacing to other fish.
Have always been kept with rays - no issues there. Seem fine with bigger fish.... I have the alpha with Brassies, a pair of Brycon, Goonch, Silver Aro and Lucy (Lacerdae).
The 4 will be added to the tank the alpha and 4 brassies are in soon....... Should be the most interesting Dorado shoal this guys ever had..... REALLY hope it works.
I personally would have very little worry about a single Hillari running a tank. They grow so slow that most of what we keep gets enough size to it fast enough that it shouldn't be an issue. My single Hillari seems much more timid then the group is. Behavior wise they seem to me to be just like Brassies.