I've introduced pair of Liopropoma eukrines, pallidum, rubre, swalesi and susumi without any problem. I've had pairs of eukrines and pallidum together fine as well as pairs of susumi and rubre together fine. One time I lost a rubre from a pair and added another one later and the new one got chased for a while.
I find Liopropoma to be the least aggressive fish and I'm confident to add any at anytime. Keep in mind they like covers so have plenty of overhangs and caves for them to feel secure. Excellent water quality and low temp is a must. If any change in water para or intro of disease they will be one of the first to die or go missing.
Here's some old pics