I personally would choose one species over two smaller groups!

say 20-25 of one species looks amazing!
You still have choise, even within the brackets of harlequins and rummynose!
Have you looked at Sawbwa Resplendens? (personally id go all males, better looking and for some reason colour up better without females). They are closely related to the rummynose...
Or with the rasbora you have the espei and the henglei (Copperline or Lambchop Rasbora)
Personally I really like Volcano Rasbora, the males far better than females but they are an interesting species and you have to work a little harder and maybe wait a little longer for them to colour up but they are gorgeous!
This is the shape of them but there are no pictures of coloured up specimens, imagine the outline of each scale in dark purple and the fish colour bright pink, like the brightest pink on a harlequin. They also get a little larger than both harlequins and rummynose at a nice hefty 2".
http

/www.google.co.uk/imgres?q=volcano+rasbora&um=1&hl=en&sa=N&biw=1084&bih=607&tbm=isch&tbnid=PsWaNAi8_5NS3M:&imgrefurl=http

/www.seriouslyfish.com/profile.php%3Fgenus%3DRasbora%26species%3Dvulcanus%26id%3D1277&docid=sktEZL94IxRrQM&imgurl=http

/www.seriouslyfish.com/images/Rasbora%252520vulcanus%252520male1.jpg&w=810&h=540&ei=DLQhT_fDJ4208QO7ndXiBw&zoom=1&iact=rc&dur=150&sig=103349512573384008980&page=1&tbnh=125&tbnw=167&start=0&ndsp=15&ved=1t:429,r:0,s:0&tx=122&ty=45
I love clown rasbora too! But never had much luck keeping them, seem very prone to bacterial problems, though maybe better in a tank than on a system i dont know! I cant believe more people dont keep them! (get to 4" though)
http

/www.google.co.uk/imgres?q=clown+rasbora&um=1&hl=en&sa=N&biw=1084&bih=607&tbm=isch&tbnid=U3-YgxK11PYMvM:&imgrefurl=http

/www.petcaregt.com/fishcare/freshwaterfishbreeds/clownrasbora.html&docid=vaSVDvsWMmjg1M&imgurl=http

/www.petcaregt.com/images/clownrasbora.jpg&w=295&h=123&ei=3rQhT-uAMoer8QPyo4DjBw&zoom=1&iact=rc&dur=92&sig=103349512573384008980&page=2&tbnh=75&tbnw=179&start=16&ndsp=20&ved=1t:429,r:19,s:16&tx=85&ty=57
Bentosi White Tip Tetra
http

/www.google.co.uk/imgres?q=white+tip+tetra&um=1&hl=en&biw=1084&bih=607&tbm=isch&tbnid=mOyq7regxDlAmM:&imgrefurl=http

/www.waysideaquatics.co.uk/index.php%3Fmain_page%3Dproducts_new%26disp_order%3D2&docid=1TxKNN_n79auOM&imgurl=http

/www.waysideaquatics.co.uk/images/white-fin-rosy-tetra-97.JPG&w=300&h=241&ei=ULYhT7n9NYrL8QOnr7CpBw&zoom=1&iact=hc&vpx=480&vpy=2&dur=438&hovh=192&hovw=240&tx=151&ty=91&sig=103349512573384008980&page=1&tbnh=129&tbnw=177&start=0&ndsp=15&ved=1t:429,r:7,s:0
Bleeding Heart Tetra (expect to pay a little more for these guys!)
http

/www.google.co.uk/imgres?q=bleeding+heart+tetra&um=1&hl=en&biw=1084&bih=607&tbm=isch&tbnid=-9mIVaSHGNbO3M:&imgrefurl=http

/dickiespetstop.com/characin.html&docid=nZFdg0ZJgewGtM&imgurl=http

/dickiespetstop.com/img0395%255B1%255D.png&w=494&h=323&ei=0bYhT8yIKMvQ8QOy5u3bBw&zoom=1&iact=hc&vpx=428&vpy=2&dur=184&hovh=181&hovw=278&tx=215&ty=86&sig=103349512573384008980&page=2&tbnh=134&tbnw=179&start=15&ndsp=20&ved=1t:429,r:2,s:15
Beckfords Pencil Fish
http

/www.google.co.uk/imgres?q=beckfords+pencil+fish&um=1&hl=en&biw=1084&bih=607&tbm=isch&tbnid=LFUqCFIJ0Q6EWM:&imgrefurl=http

/www.fishlinkworldwide.com/fish/index.php%3FproductID%3D725&docid=0JaK2kcQuuumtM&imgurl=http

/www.fishlinkworldwide.com/fish/products_pictures/pencilfish1.jpg&w=400&h=300&ei=DbghT5j8FNCf8gOIq8TRBw&zoom=1&iact=hc&vpx=464&vpy=144&dur=2913&hovh=194&hovw=259&tx=116&ty=95&sig=103349512573384008980&page=1&tbnh=126&tbnw=168&start=0&ndsp=15&ved=1t:429,r:2,s:0
Marginatus Pencilfish
http

/www.google.co.uk/imgres?q=marginatus+pencilfish&um=1&hl=en&sa=X&biw=1084&bih=607&tbm=isch&tbnid=88CNPizqJ9l0zM:&imgrefurl=http

/www.israquarium.co.il/Fish/Nannostomus/Nannostomus%2520marginatus.html&docid=yeKqkFvyZ7peOM&imgurl=http

/www.israquarium.co.il/FishPhoto/Nannostomus%252520marginatus.jpg&w=400&h=259&ei=SrghT_GKEcjF8gOGgY3SBw&zoom=1&iact=hc&vpx=438&vpy=313&dur=1212&hovh=181&hovw=279&tx=135&ty=128&sig=103349512573384008980&page=2&tbnh=136&tbnw=184&start=15&ndsp=20&ved=1t:429,r:17,s:15
Loads of choices out there! They are some favourites of mine
