On the Bentos tetra...if you are referring to Hyphessobrycon bentosi [profile: http://www.tropicalfishkeeping.com/c...entosi-190985/
] this is a very peaceful and quiet tetra. I've had them for years, and they remain under cover of plants (they do not appreciate overhead light) and in their group, not much for swimming. They will not bnother plants. Perhaps you are thinking of another tetra, like the Buenos Aires, rexpepper? [profile: http://www.tropicalfishkeeping.com/c...isitsi-190977/
]. This is a very different fish, and not one I wold recommend here in this setup.
As for mixing plant species, if you are going for a more natural aquascape, fewer species is best, as this is how nature appears. Find ones that share the same lighting requirements. My 115g has five species in total; two (three counting the floating) are spread around to create unity and cohesion, with the other two as single specimen, In smaller tanks, 3-4 species (one being a floater) will do much the same.