sorry i really don't know much about cichlids....i only had very limited experience with oscars a long long time ago...all i know is they get pretty strong when big, could get mean, and can do pretty serious damage if they want to....thats why i said they probably won't do well with the spotteds. also the food problem mentioned. aren't red devils more aggressive??
good fish to go with gars are fish compatible in size and pretty much mind their own business, like the gars. other gars would make good tankmates, bichirs would work too, aro perhaps (silver, black), and i'm sure there are catfish that would work but i don't know specifically what (i know RTC and pleco would NOT be good). im sure there are cichlids that would work but again i don't know what specifically.