to your second question, males have more bluish/green spangling on their bodies. females generally have the blue marking only on their cheek. the rest of their bodies are a greyish black color - usually.
1 jd and 1 severum should be fine in a 55g. As long as the jd is not that aggro. Males dorsal fins are more pointed and red on the tips female dorsal fins are more rounded.
I'd say yes if they're both females. A male jd may cause problems, but if you have an extra tank it's worth a shot.
Male jd are nicer/more colorful imo.