As far as adding goes... well, improving first might be the best answer.
White Clouds are coolwater fish, but they can tolerate the lower end of the tropical temp range.
Corydoras should not be kept singly, 3 is a bare minimum.
The Chinese Algae Eater will get rather large and territorial as it matures. It will also not eat algae (unless very young) and will be a rather heavy waste producer.
Angelfish can be quite territorial. (Today I had to move one of mine because of territorial issues. I had 4 in a 100g tank)
Tiger Barbs should be kept in groups of 6 or more, and preferably in a species tank. They will chase and shred fins of anything they can catch, like the Angel, Platy, and Rasbora. They are terrors in small numbers - a bit less so in large enough groups because they pick on each other and leave the others alone a bit more.
Platies can be kept alone, but do better in groups. Not mandatory though.
A 23g tank is borderline for an Angel (long term), but...
I would keep the Angel,Platy, Rasbora, and the Cory.
Add 2 Corys
Add 6 Rasboras
and if the tank is established and has a bit of algae growth to support them, I would add 3 Otocinclus. They will control algae, but if you add them before there is any they will starve.
[EDIT]
By the way... Plants absolutely, positively, and without a doubt do not create algae. That is ridiculous. Actually, plants will outcompete algae for nutrients and reduce the growth of algae in a tank.