Hi,
Please go through the contents of the above link: https://www.keka.com/library/minimum-wages/tamilnadu.
As far as I know, there is a strict rule prescribing any percentage of HRA as part of the salary under TN Shops & Estts. Act. Even under the Minimum Wages notified by the TN Govt., there is no such stipulation for HRA, but there is the Basic and DA part of the minimum wages, with no HRA stipulated.
However, any attempt affecting the present emoluments will be a contentious issue, and HRA is one such matter. An attempt to reduce the percentage thereof will not be acceptable by the employees. Perhaps there should be an agreement or Letter of Appointment where these rates could have been mentioned. If so, it's not possible to effect a unilateral action by the employer without the formal approval of all employees concerned.