Anne,
how will you make different weight mice? I suppose you could try to fix a little bit of blu-tack into the inside of the cone, but I’m not sure there would be much difference given how small the rocket mice are.
Different masses and positioning of the mass on a straw paper rocket is something I do with TY students – and for them the science is a bit tricky.
You can read this teacher level article about how mass and rockets interact and pick up some student centred activities from this great Rockets Educator Guide.