Bettersense: First, welcome to the interesting world of pinhole. You may find it taking more and more of your photographic time. Follow your passions.
As for film flatness, it not only is not an issue, but there are some unique things you can do with pinhole regarding purposely unflat film, such as anamorphic cameras (film wrapped in a cylinder, the pinhole at the end of the cylinder); curved film planes, etc.
There is a slight "defocusing" effect when using a pinhole camera for extremely closeup subjects, an example being when the subject matter is the same distance as the focal length of the camera, in which case the image blur at the film plane caused strictly by geometric effects is double the diameter of the pinhole itself. When I intend on shooting dioramas or other close-in subjects, I use a camera with a purposely smaller-than-"ideal" pinhole diameter.
The quality of the pinhole is at least as important as getting the "ideal" diameter. Actually, there is no "ideal" size from an image aesthetic perspective; it's all good. Smooth, thin-walled pinholes are important, whether hand or commercially made, as they make for smoother tones.
Paper negatives: I've shot paper negatives for the last 15 years, both in glass-lensed and pinhole cameras. The biggest problem I found was high contrast in bright light. The problem with MG paper is that the high-contrast part of the emulsion is activated by the UV/blue light, which predominates in daylight. Therefore, I've chosen to use grade 2 paper; I like RC finish for negatives, as it sits flatter during contact printing, so I use Arista's RC grade 2.
Secondly, I find with pinhole cameras that you can lose shadow details in daylight when trying to keep the highlights from blowing away, so I've taken to preflashing the paper in the darkroom prior to loading in camera. I use a faint, 7.5w type S-11 bulb, suspended about 30" above the work surface in a metal soup can enclosure with a 3mm aperture; typical preflash times for Arista grade 2 are 10 seconds. This yields a faint, gray tone to an otherwise unexposed negative. It helps to bring up shadow details. I actually preflash all my paper negatives, even when used in glass-lensed cameras.
Good luck and have fun.