Showa Gams only has cheesy flamethrower breath, and King FooDog didn't burn to death from MG's attacks (which would be quite a bit hotter). FooDog was quite willing to ram into MG, so ranmming (or being rammed by) the turtle's shell shouldn't be too bad for him.
Showa Gamera could be hurt fairly easily for a daikaiju. (I think the vulnerability combined with spirit was part of Gamera's appeal, actually.) Thus, the big foo could scratch him up and such. On the other hand, scratches aren't usually lethal.