The Thread fin Acara (Acarichthys heckelii) was indeed part of the Geophagus complex before being moved to the genus above. These are just as steady as the Geos and in the short term should get on just fine. However once the motoro reach 12-18" disc the situation becomes very precarious, Rays are perfectly designed to take such fish down, once under the powerful disk there is no escape.....trust me its happened to me many times over the years, a hungry gravid female is a very different proposition. As with all fish so long as you keep them well fed they tend to leave tank mates alone, they basically lose the will to hunt, why bother if their food is served on a plate every day......you go on holiday for a couple of weeks however leaving the capable friend looking after them on the reduced diet to ensure they are still fit and healthy on your return and its a case of who'd have thought it!
You need to look at the big picture, the long term, choose fish that will grow with your rays, Geophagus altifrons, True Parrots (Hoplarcus psittacus) Twin bar Tinfoils ( Barbonymus schwanenfeldii), Wide bar Myleus (Myleus schomburgkii) etc.
In very large tanks such as Jim Barry's for instance with plenty of hideouts and copious amounts of food going in every day the risks are vastly reduced, it can work and it does obviously work for some but its a very fine balance.
Good luck with what ever you decide.