A colony of like 6 Shellie's would be great in a 20. I'd just keep them alone in there. For the 36 you really aren't able to do much other than a pair of apistogrammas ( many awesome colorful types available) or rams. You could do a colony of saulosi. Like 2 males and 7 females. That would be great color and they're awesome, beautiful, and entertaining! One of my favorites! Here's my male and females
I've never tried live plant s. just plastic. Sorry