Comparing PSNR/SSIM of two videos with different fps/bit-depth
PSNR and SSIM are image quality metrics. Their use on videos involves calculating the metric for each pair of frames and then applying some aggregating operation to the series of frame scores, typically the arithmetic mean. What this means is both inputs must have same frame count with each encoded frame paired with its source frame.  So, deterioration of subjective perception caused by downsampling the fps isn't measured. For a valid and useful comparison, your best bet is to downsample the source to the same FPS using the same methods as used on the distorted video before carrying out the comparison.

As for the bit depth, both operands must have the same pixel format of which bit depth is one factor. I would recommend converting both inputs to the higher bit depth. This will allow you to preserve the details in the distorted input if you applied any filters to it after the bit depth increase.