In captivity Armatus will get twice the (size 24"or more) of the common 12" Dats (indo + silver) and grow faster. With any predatory co-hab it's usually trial ,error, and luck nothings written in stone. I've done it with all 3 species of Hydrolycus however the most successful, longest lasting Dat - Vamp combo was the smaller Payara - Scomberiodes.
Dats ignored the Payaras and visa versa. I've had more issues co-habing multiple Payara and growing out small Payara in small tanks (less than 100gal) than Co-habing w/ other fish large enuff not to get eatin'.
On a side note - I got slamm'd here on MFK for growin' RBP's in 29 gal. - a few years later - 9" Male RBP, 10" Female RBP, 12" Caribe in 75 gal. - Trial, error, and luck.