Yes, it is true that by protecting the tigers, we protect all other forms of flora and fauna. Tigers are known as umbrella species. It is a term used for species whose conservation and protection would indirectly lead to the conservation of other species of flora and fauna.
Protecting the tiger means the flora and fauna of that region also gets protection and conservation.
No, a tremendous increase in the tiger population is not desirable. A tremendous increase in the population of tigers means, the food chain gets disturbed and poses a threat to the existence of other animals.