It has been a long perpetuated myth that fish only grow to the size of their tank. If you do routine proper maintenance and water changes certain species can and will outgrow their tank. They will also be deformed and eventually succumb to disease if kept in too small a tank for an extended period of time.
Algae eaters are not so much a necessity as they are a convenience for cleaning up algae, you can certainly clean the glass yourself, and you can select other species that wont grow too large (bristlenose pleco for example are a dwarf pleco that wont exceed 5 in on average) It is an easy mistake with the common pleco as they are sold when they are very small and with no warning as to how large they will get. They can live 10-15 years. My mother in law has had one for seven years. They are far better suited to a pond than a fish tank in my opinion.
Goldfish and cichlids are not usually good tank mates because they require different conditions (goldies like cooler water than cichlids) and the goldfish are considered messy fish in terms of food consumption and waste production so they can require more maintenance.
stands for Local Fish Store and can be anything from petsmart to your local mom and pop aquatic store or pet store that also sells fish. Most of the Mom and Pop type shops will take your fish and often give you store credit so you can get food or other fish that you do want.
You could almost certainly trade in the cichlids for any other fish you so desire.
You might also want to research goldfish if that is your fish preference as I believe there are some that wont grow quite so large and would be quite happy in your tank.