Yes I also got to know from various sources ...But can I keep betta and shrimps both....One more doubt is that can a small aquarium be converted into natural aquarium
They can, but you would have to follow this process:
Choose the correct shrimp for your tank, most likely a color variant of the neocardina (cherry shrimp) type.
Plant the tank from day 1. The more plants the better.
Get the plants growing nicely while the tank cycles itself. To aid in this you can dose the tank up to 0.5ppm ammonia, let that fall and repeat a few times while the plants grow.
After the plants are growing nicely, make sure ammonia/nitrite is 0 and nitrates are below 20ppm.
Add 10 or so shrimp. After a few months they should have bread enough to support a nice colony.
Add a single betta and hope he doesn't constantly chase shrimp. He will definitely eat a few juveniles, adults should be safe, but there should be enough survivors to keep the colony alive.
Plants that will work in a simple low-tech 2.5 gallon:
Crypt parva
Crypt lutea 'hobbit'
Rotala / ammania 'bonsai'
Anubias nana 'petite'
Possibly alternanthera reineckii 'mini'
Pogostemon helferi
Bacopa sp. 'compact'
Blyxa japonica
Littorella uniflora
Bucephalandra
Lobelia cardinalis
Any moss -- Java / christmas / serpens / fissidens etc