I would mix it any day. You need a bigger tank though, dont get any more fish till you have a larger tank around, even a 55 for starts would be good, I don't see home fish can even be kept in anything that small anymore. 30 gallons is the smallest tank for about anything. And its my feeder guppy tank.
At this point you need to focus on a larger tank and not new fish. If you already have a JD, a firemouth, and a Cichla in a 20 gallon tank, I'd say it's enough for now, even if the fish are small. I wouldn't throw a frontosa into the mix.
I've mixed Frontosa with CA/SA cichlids before, they actually match pretty well providing you don't have too aggressive species and some space.
I currently have African, madagascan, and CA/SA together with no issues. It's all about knowing what you're mixing and how they will react. do some research or if you've kept the fish before you will remember their behaviour and compatability.