It will work. Especially if you are going to get a bigger tank later. 29 is too small for a Jack Dempsey as they grow. But 55 should be alright. You have some time until he is the size that would require the 55 but just get it as soon as possible. If a fish stays in a tank that is too small for too long, it will permanently stunt its growth.
As for mixing Africans and South Americans, some people get all huffy about it, and I guess i can understand why, the thing we all really need to understand is that we are all in the fish hobby for different reasons. Some people like to breed and create as natural of a habitat as possible (which is awesome), and some people just like to have a lot of bad ass fish to look at (which is also awesome). As long as you keep fish healthy and happy (which can most definitely be done while mixing African and South American), then who cares what you do. We have fish to enjoy them! So enjoy them!
And yes, your JD and EB will be fine if you follow the suggestions already given, introduce them to the new tank at the same time, give them places to hide and objects to use to establish their own territory. They're gonna be great!
But you have to start somewhere gl
