If the clutch is slipping then the friction discs are the usual culprit along with weak pressure springs - we would suggest however that the Pressure Plates are replaced at the same time. Interfacing new parts with old items is never good practice.
Why not order all the gaskets you require at the same time - save money on shipping costs with a multiple order.
Try this gasket sealant: 08C70K0234M - as used by factory professionals!