On the issue of adding another fish, I certainly agree with burnsbabe and jeaninel that this is not advisable. I can almost guarantee health problems for the fish if you add any more to what I consider to be an already over-stocked 29g, and I would take measures to improve things for the present situation.
Many think that because a fish is now small, it can "manage" in a smaller tank than what is needed for the mature fish, but this is erroneous thinking. As a fish grows, which it does all its life unlike us, the external skeleton is developing as are the internal organs. The size of tank significantly affects both of these developments, from the aspect of physical size but equally the water quality. In their natural habitats, these fish would never be in such close proximity to each other, and forcing them contrary to their natural tendency takes its toll. Fish release substances such as pheromones which no amount of filtration can remove, and the fish will be stressed in such close (to them) confinement. And this means a weakened immune system that will lead to additional health problems that would otherwise not be an issue, and if really bad, deformed internal development called stunting.
Another fallacy many hold is that adding more filters somehow makes it better; not necessarily so. Filters move water around, and bacteria colonize the media to handle the nitrification issue. This aspect has no bearing at all upon what I mentioned in the preceding paragraph. Chemical filtration (carbon, etc) has some, but this is very limited. Mechanical removes suspended matter from the water, but this does absolutely nothing for the fish. "Clear" water is not "clean" water chemically. We can't see the urine and pheromones, and they are still there until we do a major water change. This is why discus breeders can grow a group of discus fry is smaller tanks--they perform several complete (100%) water changes each and every day. This is solely to deal with the unseen "stuff" like pheromones that affects the fish and inhibits their proper growth and internal development.
The existing cichlids in the 29g are sending out signals continually to each other, and this is affecting their growth, behaviour and internal development. While a 55g may be planned for the Spring, until it is a reality I would not add any more fish to the existing 29g, and I would be vigilant in several weekly water changes.
Last comment on the algae eater; I assume this is the "golden" variant of the common Chinese algae eater, Gyrinocheilus aymonieri.
There are three species in the genus, the latter is the one usually seen. All attain 11 inches, rarely if ever eat algae once they leave adolescence (and often long before this), and they are aggressive to each other and other fish in the tank. There are confirmed reports of these fish eating the scales of larger fish. They are not good community tank fish. Even at 2 inches, it is possible that the survivor was responsible for the death of the other. These are also very messy fish waste wise. But the "aggressive" signals they send out as they mature will affect the other fish, even if no physical "attack" is present. Another case of the chemical signals fish use.