<< can i place space shims between rough sill and the bottom of the window? >>
Read the directions that came with the windows, then do it as suggested. If you
lost the directions, get a new copy. If none were included, go to a Pella
window store and borrow theirs. Or ask your dealer to photocopy the directions.
<< should the shims be placed only at the corners? >>
Since the corners are the sturdiest part of the window, logically, yes.
<< once the window is square and nailed in place, can i remove all the shims?
or should i just leave them alone or do I need to nail the shims as well? >>
Leave the shims in place and fill the spaces with latex foam like the DAP
product. Be sure to use a sealnt under the nailing flange if your instructions
call for it...most do. Good luck.