Since the tank had only been running for 2 weeks before you got the first fish, that is not long enough for you to have done a fishless cycle using ammonia or fishfood. You are therefore doing a fish-in cycle. You need to test your tank water twice a day for ammonia and nitrite, and do a water change every time you see a reading for ammonia, nitrite or both. If you don't have a test kit yet, do 50% water changes a day till you get one. Liquid testers are more accurate than strips (and strips don't have an ammonia test on them so you'd have to buy one separately). More details
hereYour plants might help a bit, but you'd need a
lot of plants to eliminate the need for water changes. (Think underwater jungle)
Once you have zeros for both ammonia and nitrite, it will be OK to get more fish, but only a third of the fish already in the tank on any one occasion. The filter bacteria will catch up to that size addition quickly, but more fish than that could lead to an ammonia and/or nitrite spike.
My only concern about your fish choice is that the barbs and tetras might nip a betta's fins. They are not known for being very nippy fish, but some species just can't help themselves in the presence of long flowing fins. Would you consider something like a pair of honey gouramis instead of the betta?
In a 54 litre tank, I would not have more than 2 shoals of fish. The shoals of just the barbs and tetras put you at three quarters stocked. Adding a betta pushes the stocking to almost it's limit. A pair of honey gouramis instead of the betta woud take to to completely full, but it would be do-able if you did large weekly water changes without fail.
There are no algae eating fish suitable for a 54 litre tank, I'm afraid. The only small algae eater, otocinclus, needs to be in a shoal, but your tank is too small for a shoal and would probably not be able to grow enough algae to feed a shoal. Besides, with the barbs and tetras, there won't be enough room left even without a betta/gouramis. The other algae eater I would normally recommend, nerite snails, would not be a good idea with assassin snails in the tank.
You do not
need an algae eater. A well run tank shouldn't have much algae. It is better to work out why a tank has algae and sort out the cause rather than buy an algae eater.
Edit to add:
The community creator can be a bit hard to find. Click on fish profiles in the menu at the top, select any fish and scroll down to the bottom of the profile. Then just enter your tank details and add fish. You can save your virtual tank but you do have to register and log in separately from the forum.