It depends on the capability of the company making the boards. You would need to have less than 6 thou for the design rule to avoid the DRC error but that is smaller than common board clearances. If you want to change the clearances to remove the errors then go to DRC/creanaces and edit the 7mil to something less then 6mil. Most of them are pad-pad errors.
If it was me, I would change the package. The pads are too wide for SSOP20 and in fact the pitch looks slightly wrong.
Keith.