Tamil Nadu intercaste marriage benefits query

Just call on Social Welfare dept personally, with photo identiies and marriage certificate and file application.  If you are eligible they will pay the amount or return the application quoting such reasons for rejection.  (In your case as castes are the same you are not entitled for benefit, but make a try, as there is no financial loss to you in filing application.

since you both belong to the same caste you cant claim any benefits from the government

Yours is an inter-religious marriage which is also like an inter-caste marriage. You can certainly try. Your success would depend on the prejudices of the person/authority deciding just as it is here. If you are not successful you can go to court. Are you claiming inter-caste marriage benefits or benefits of reservation for lower castes? The criteria for the two are different.