I love my harlequins so i'll back that up, I have a school of a dozen in my community tank and they are the most active little fish you'd ever see. Softwater is liked by them though
tetras are super easy, neons or cardinals, glowlights, rummynoses, black tetras. get them in large numbers. about 5 or more. the more the better. rummynoses and black tetras school in larger numbers so they would be great.
Assuming tank specs are correct... I'm finding Norman's Lampeyes to be the hardiest fish I've ever had. I've only had to euthanize one ever (that one came to me with wasting disease)... all the others are thriving and seem to have amazing health. I am told this is also due to them being a lot less popular and thus not over breed.