Can I Print Only Some Rows or Next Month's Tasks of My Project?

Sure. In Merlin Project you may enable a filter, limit the visible date range or use the Details window before printing.

Filter

Limit Date Range

Details area

Enable the details and select the same view configuration as in the main part of the window. Select in the main view the part you want to print, see the 'details' view showing selection and sub-structures. Click any row of the details and print from there.