Yes SC/ST person can only file a case under the Atrocities act.

Non SC/ST person cannot file a case under the act same is non maintainable.

Court complaint files by effected person only. Rest, any person can inform the police abut commission of cognizable offense.  And police can file FIR.

affected person must file complaint for offence committed under Sc / St act 

1.  ONLY "affected" and "aggrieved" SC/ ST person can file case in court.  That is the law.

2. ANY other person (even though an SC /ST) who is not affected or aggrieved CANNOT file case as a Complainant.  Third party filing criminal cases on behalf of somebody else is legally barred.

You have to file a petition in High Court for   direction in this regard based on your case.

As such the affected person must file an First Information Report (FIR) in the Police Station of the area as per relevant provisions under Chapter II of the Scheduled Castes and the Scheduled Tribes (Prevention of Atrocities){PoA} Act, 1989.

The person other than the person who do not belong to the SC/ST community cannot be said to be affected by the atrocities committed against the said community person, hence there can be no complaint entertained by the police from a person who was not affected particularly, however an information may be passed to the concerned police about the commission of such an offence.

Person affected by the practice of untouchability may approach the Police Station covering the area of such incidence and register First Information Report(FIR) as per provision under Sections 3 to 7 of the Protection of Civil Rights Act, 1955.

Section 3- Preventing on the ground of untouchability

Section 4 - Enforcing on the ground of untouchability, any disability with regard to-

Section 5 - Refusing on the ground of 'untouchability'

Section 6 -Refusing on the ground of 'untouchability' to sell any goods or refuses to render any service to any person at the same time and place and on the same terms and conditions at or which such goods are sold or services are rendered to other persons in the ordinary course of business.

etc.

Yes it is necessary that SC ST case should be filed by SC ST.