Hi there welcome to the forum!!
Just so I can get a few things right from what you have said, this tank is second hand and you have just re set it up with the fish etc? If this is right it should mean your filters are still live and healthy.
At the moment it sounds like you have 2 silver dollars and a red tailed black shark. The silver dollars are large shoaling fish getting to between 6-8 inches each so ideally you need to add an other 2 Silver dollars so you have a group of 4.
Bala sharks again are a large schooling fish and are for big big tanks usually 6-7 foot plus in size as they each get to a foot long and are very active. A good alternative to these are Denison Barbs (bit more attractive IMO) but again these need groups and I think you should choose between these or the dollars as other wise your tank will fill up very quick.
The red tail black shark is a good fish for your tank, it can get a bit aggressive but should be okay depending on what you go with in your tank.
Your "common algae eaters" are they plecos? Or are they something different if you can get us a pic of something similar from the net we can help you out a bit more. The bad news is that most of the common algae eaters are quite bad aquarium inhabitants with things like the common plecos getting to over 12 inches very quickly which is wayyy to big for a standard 55g then you get the chinese algae eater which gets aggressive as it gets older and same goes for the golden algae eater however there are a few others that could work for example flying foxes or siamese algae eaters. Common names like "algae eaters" is used for tons and tons of fish that range between 2 inches and 2 foot.
The food you have is pretty good for them, if you inter space the feeding between the 2 types they should be quite happy. Though silver dollars are more of a vegetarian fish so would probably prefer the flake.
As for decor, what ever you like

I pack my tank out with branchy twiggy wood and smooth cobbles with valis which is a long reedy type plant. Other people put spongbob squarepants ornaments.
For fish ideas my tank is quite similar though a bit bigger than yours but similar fish could work in your tank have a look in my sig
Hope thats helped
Wills