MSCA general: Is it possible to recruit researchers under a "working contract"?

This depends on the exact type of contract. Please refer to the MSCA-model grant agreements (art.6/6.2/A(b)(i)), where it says: ...be recruited by the beneficiary under an employment contract (or other direct contract with equivalent benefits, including social security coverage) or — if not otherwise possible under national law — under a fixed-amount-fellowship agreement with minimum social security coverage.