Is the diamond head tetra a color variant of neons? That is all that came up when I searched it.
I think any of those are suitable for your tank but not all of them as that would overstock it since the tetras, rasboras, and danios all need schools of 6+ of their own kind.
Celestial danios stay quite small and can be really skittish so you may want to pass on those depending on what other fish you get.
Killifish can be nippy but I think it'd depend on which kind you get. Are there any particular ones you were looking at? There are tons!
For a stocking plan maybe a trio of male guppies, 8 harlequin rasboras, and something for the bottom level of tank perhaps? I'm not the best at stocking plans so someone may have a better idea than me.