Bolivian rams would be a much better choice than kribs for that tank I would say.
They are hardier than blue rams too I believe, try to pick out a pair at the LFS otherwise you may have some aggression problems - not easy to sex but observing behaviour is the best way. I have two and love them, grow to a nice size too but not too big for your set-up.
I reckon with good filtration and maintenance schedule in an established aquarium you could have your corys, a pair of BN (with plenty of hiding places and some bogwood), harlequins and rams plus something else. Either you could up the number of harlequins and corys to about 8 OR add another shoal of 5-6 tetras or the scissortails you suggested, something for the midlevel of the tank I would suggest, as it may end up looking a bit bare. This is very similar stocking to a 21 gallon I had set up a few months back and worked well.
Hope this helps