Aquarium Glaser notes:
"10. July 2017 For the first time ever we can offer this pretty and very interesting tetra from the Congo. The species is practically unknown among aquarists. The genus Monostichodus (formerly: Hemistichodus) belongs to the relationship of the subfamily Ichthyborinae (fin eaters). Many members of that group are specialized fish that feed almost exclusively on parts of fins that they bite of from other fish. At least M. mesmaekersi seems not to do this. We accompanied fantail goldfish, bettas, killifish, and knifefish with the Monstichodus, but the fins of all tankmates stayed unharmed as well as the fins of conspecifics."

and:

"They only report known to us about keeping the fish says that they proofed to be scale-eaters. We can neither agree nor deny this. What we can say is that the fish listed above also weren´t attacked for eating their scales. In our tanks the fish feed on live food, basically live bloodworm. The maximum length of M. mesmaekersi is about 6 cm. It is very interesting that every individual fish seems to have a slightly different pattern on the caudal fin. In any case this beautiful tetra is a challenge for every enthusiast that likes to be a explorer. The fish are not shy at all and their behavior is much alike that of pencilfishes (Nannostomus) from South America."

One can assume if the fish was very hungry then there is a chance the long fins of a Betta may not stay intact, ditto for others such as angelfish, guppies or anything with long flowing fins. Presumably when other food is abundant that would be the first choice for the fish but until proven otherwise prudent dictates at this early stage to be aware of the potential for damage here.