I was thinking to start my campaign as Bengal and since i'm new player to all of this i have a q. Will forming Delhi prevent me from achievment? Forming Hindustan won't prevent achievment, will Delhi does anyone know?
You can form any nation and still get the Bengal Tiger achievement. You just have to start as Bengal.
If you want to know if you can get an achievement after forming another country, have a look at the achievements list in the wiki. If the country is in the "Starting conditions" column, you have to start as the country. If the country is in the "Requirements" column, you have to be that country at the moment that you get the achievement.
Also, don’t trust the wiki 100%. It is pretty reliable, but in the past it has had incorrect requirements.
Make sure that, when you do form a nation or do something else that you aren’t absolutely sure is allowed, you back up your save, check the achievement is still available, and be prepared to Alt-F4.
Also the bengal achievment is best done by bee lining through the hymalaya. Saves you time. Hindustan is better to form with dehli you can get 2 achievments like that. Or Bahamanis with tales of 2 famalies.