I support Mr. Saikumar's presumption that the T.N.S.E Act, 1947 is applicable to your establishment, provided that the premises carrying on the activity of designing is not a notified factory under Sec. 85 of the Factories Act, 1948 by the Government. Having said that, I would like to elaborate on Saikumar's views a little more academically.
Basically, a contract of employment, I think, is a species of the genus of 'the contract' under the Law of Contract but with the exception of the implicit possibility of creating rights and obligations beyond the terms of the contract. In order to circumvent this, novel methods such as contract labor, fixed-term employment, etc., are employed by certain employers worldwide. That is the reason for the non-emergence of a Comprehensive Labor Code applicable to all systems and ventures of employment.
Specific Query on Establishment Definition
Coming to your specific query, yours is an 'establishment' as defined under section 2(6) of the TNSE Act, 1947. Section 2(12) of the Act defines a "Person employed" to mean a person wholly or principally employed therein in connection with the business of the establishment. When machine engineering designing is the business of your establishment, the people engaged to do the job in the same place will be the 'persons employed' irrespective of their mode of employment. However, their eligibility for the benefits you've mentioned is dependent on the tenure of their contract with your establishment.
Regards.