shouldnt realy be combining them but new builds dont seem to worry about it, yes to pipe sizesi would run it all in 2 inch and reduce close to bath and run 1.5 up to the basin reduce close to trap
you could run 2" , i would just use 1 1/2" then reduce to 1 1/4 on the elbow comming through the stud or what ever to basin, you might need an anti vac trap on basin,
It's not ideal but sometimes builders put soils into stupid places and bossing 2 wastes out can be impossible. You can consider short boss in soil boxing and run up into floor? Either way stick to max lengths for each size.