Hi Laura,
I don't think it is possible to assign a MRP profile directly to a Material Type.
Maybe you can try to add a logic using BAdI: BADI_MATERIAL_REF.
You can also define defaults for each user with transaction code OMT3U, but it is not what you are looking for because you can not link the MRP Profile with MRP type, so I would try with the BAdI.
Kind Regards,
Mariano