LoL that's what I was hoping not to hear... I have 18 of them and hopefully if everything works out I'll be adding 12 more.... I'm thinking the school of them will deter the rays a bit from going after them.
If these disc fish are possible victims I'm sure the g.altifrons and guainacara. Steirgosi are goners....
The Tinfoils (twin bar) go for less than £5.00ea as youngsters and are the better option in the long term due to the eventual size of 16", having kept two very large groups of them over the years I personally wouldn't keep them with my rays unless I had a 1000 gallon plus tank for them, in tanks 500 gal or less they put too much strain on the water quality due to their insatiable appetites.
Geophagus altifrons is the largest of the Geo complex (depending on locality) but even these would need to be very large to stand a chance, they are more trusting than disc Characins and tend to hug the lower regions of the aquarium making them easier prey. Guyanacara are out of the question,I wouldn't even keep those with adult Hystrix....too small!
Always nice to see your setup Ade....beautiful tank and fish.