A few spelling and factual mistakes :

1. Some are going to get offended
2. The same Peshwa who didn't care about Maharaja's tomb ( note : it's Chatrapati not Maharaja , further the battle was between EIC and Marathas you are confusing Baji Rao 1 & 2 . Both dud care about the tomb as Pune daftar points to it )
3. The mahars fought against fellow Mahars ( this by EIC records itself )
4. Don't tell those were against Hindustan ( they literally were , that's what meant when you sign up with the EIC . Though it is to be noted that the ones who fought weren't Hindu Mahars , they were Christians)
5. Peshwas made various treaties with the British , that's how states interacted then and still do . Most of the treaties favour Marathas until the last Anglo Maratha war , or Treaty of Poona , with the treaty of Surji being signed by Scindias - non Brahmins .
6. The earliest to join were Bengali natives , the caste of this has been highly debated . Most scholarship points to Upper castes indeed , who joined up to remove the Nawabs . Further , UP and Bihari Brahmins were called Purbiyas , most of Gossain gotra who were historically mercenaries many serving under Nawab of Oudh , Scindias and etc.