30g is to small just for one jack dempsey, jd's get between 8"-12" ive seen some huge male dempseys. I would recommend a bigger tank for one, and as for mixing african with central american cichlids some people do it, but i prefer not to.
the only jack dempseys that will max out around 6-8" are the Electric Blue Dempseys, they are the only known Jacks to stay that small. All other Dempseys can reach 12"
I had a male Jack Dempsey for 7 years who recently passed away that was over 1 foot in length. No way can you keep a JD in a 30 gallon with other cichlids. IMO a 55 gallon for one JD is as small as I would really go.