SCOTUS was given power to create FRCPs for federal district courts under the Rules Enabling Act, 28 U.S.C §§ 2072-74.

To be valid the FRCP must be within Congress’s power to regulate the procedure of federal courts (Congress cannot delegate to SCOTUS power it does not itself have). So an FRCP would be invalid if it was beyond Congress's power to regulate federal proecdure.

In addition, in the Rules Enabling Act, Congress put a limitation on SCOTUS's power to create FRCPs. 28 U.S.C § 2072(a) states that a FRCP cannot abridge, enlarge, or modify any substantive right.