only other knife that I can think of that get over a foot and aren't common or super aggressive is the "gold line" knife fish... S. macrurus. but idk if you can keep multiple of those as we don't in our lab.
BUT, we do mix those with other south american knife fish. In one tank we actually have glass knives (Eigenmania spp), black ghost OR brown ghost (don't mix those two usually), and a gold line.
oh yeah, also there are R. rostratus... we keep thsoe together alone with gold lines and we have a few pushing 18 inches at least.