Thanks for using ngx_pagespeed. Next time you may write to "ngx-pagespeed-discuss@googlegroups.com" for quicker response.
I understand that PNG8 is more effective in reducing bytes, as compared to PNG24. Images with 256 colors or less will fit perfectly in PNG8. However, images with more colors would have degraded quality if they were converted to PNG8.
ngx_pagespeed does not convert PNG24 to PNG8 to avoid the potential loss of quality. And I'm sorry that currently there is no way to force such conversion. However, ngx_pagespeed keeps improving the image compression performance and may do the automatic conversion for 256 colors and below.
If your images don't have alpha channel (transparent color) and you are okay with lossy compression, you can turn on the "convert_png_to_jpeg" filter. This filter will convert PNG images to JPEG if the latter has few bytes.