from what i can tell u could add alot more to that tank. i would maybe double the tetras of one of the specias to have a nice shoal. .. but others may disagree
5 Harlequin Rasboras and 3 Scissortail Rasboras for the tank, might go for another 5 Harlequins and maybe ditch the tetra idea, and get a few more Scissortails.
Its my opinion that a large school of one type of fish is more impressive than 3 or 4 smaller schools. I'd keep to the harlequin and scissortail rasboras like you say, but increase both the harlequin as well as the scissortail numbers.
They actually look more like false harlequin rasboras but they are pretty! I'll get another 5 next week! they were just under 2 quid each, not sure if that's a good price for them! the scissortails were £1.30 each
Well this morning i came down to find only 3 false harlequins left in the tank, after moving many rocks, and plants, i look in the filter.. and yup, there is two of them happily swimming around in the filter! nutters.
I saw some new Neons recently something like headlight tetras (can't remember exact name) like Neons but smaller and they had intense blue / green reflective areas round the eyes. I had already set my mind on Cardinals otherwise would have got these!