it would be wise on the second floor just to do it right and not
fool around cutting corners....you might pay for it later...
1 1/2 will barely work on a shower situatioin
although I have seen it done that way many times
in many older homes that was what was used.....
but if you have got the walls open , its not that much more trouble to
drill 2 inch holes and not worry about the shower drain getting overwhelmed
someday in the future.....
yes, you really should install a solid pvc trap on the second floor
Wether you are capable of hitting the measurement perfect on
that trap is the big question
if you measure it perfect to center and use the type of PVC shower
drain with the rubber neopreme gasket in it you can glue everything
together but the stand pipe comming up through the floor
of course you can try this without useing the glue to make sure you got it right first.... then mark all the fittings with a majic marker so you stay on the money... and do the final run with glue....
you can set the base in place then measure the stand pipe and
glue it in through the drain hole once the base is in place and you are sure about everything being perfect....if you so wish , that makes things easy
then install that neopreme gasket and tighten it down....
good luck , have fun