changeing soil

Right now I have my girls in 3, Jeffy #5 Peat Pot filled with Miracle-gro seed starting soil. When its time to transplant my girls I was wondering if it was alright to change soils, in this case FoxFarm Ocean Forest soil, and will it be bad to have MG seeding starting soil with FF Ocean Forest soil in the same container?

you can proabably transplant when they are about 2-3 weeks old
whatever you are comfortable with
you will know when they are ready