in a 29, with decent sized cichlids (think convicts) I'd only recomend doing a single pair....anything more than that would be pushing it imho.
However, look at my 75 gallon thread. I have a trio of rainbow cichlids (stay under 6 inches) and plan on having another cichlid pair and getting some more angelfish as well. You can totally have a planted tank with smaller cichlids.
In fact, I don't see why you don't just plant both tanks? I can easily suggest some cichlids that would work well with plants for you if you'd like...and I feel like most cichlids that are under 6" can work very well with plants as well.
for example, you could do your 29 planted with maybe a pair of HRP or rainbow cichlids or keyholes or something, then in the 55 have a mob of little fish, or do the 29 for little fish and the 55 for plants and certain cichlids that could work with your desired level of planting. For example...I could see you doing something like a pair of HRP with a pair of rainbows in a 55, then have some sturdy dither fish of your choosing, like larger bodied tetras or barbs or other schooling fish, and call it a day.
And I'm not even saying that cichlids need to be kept in low tech tanks either, you can totally have cichlids with plants in even a densely planted high tech tank (what I'm doing), its not just all java fern and anubias with them.
So really, to make this easier, decide on what kind of cichlids interest you, and then I can better recommend ideas to you about that.