I might were it me, Look to tetras that get a bit larger such as columbian tetras or bleeding heart tetras. In my expierience with rummy nose, they are sensitive to any and all excess dissolved organics. They might also look like a snack for the Severum
. Severums usually consume a considerable portion of vegetable matter (plants) and it is needed for their longterm health. I have two that enjoy romain lettuce hung from veggie clip, spirulina flake, and regular Cichlid pellets and other foods. If you kept it well fed,, It might not pay any mind to the rummy nose. Maybe start with six and see how they do with the Severum
. Should you decide to place another Severum
with the first,, They are in my view,, pretty particular. If the second fish is not to the first fishe's liking,, It will become clear and steps to move one or the other would be in order to prevent constant chasing or harrassing. To get a pair,, Would prolly be best to purchase five or six juveniles ,and once a pair forms,, rehome the others. I like your tanks stocking plan and hope the pleco is a small variety such as Bristlenose. Good luck.