I would like to ask if it is possible, for instance in monthly cups, to remove the constraint of starting points for the penta version.

I mean, since we are using the pie rule, why would we have starting points, 1st player could put one piece for each color everywhere on the board and the possible openings are far bigger now.

This is something I have been wondering as well and would definitely be in support of. One of the reasons why Gomoku 9x9 has very interesting games is because there are so few limitations on the starting positions.