Image compression plugin required or not?

Thanks for reaching out @raval104. See below answers to your queries:

Q:1, I am using the shortpixel optimizer plugin to compress images. I just installed this plugin: Morden Image Format, and it converts images to WebP or AVIF format with a much reduced size. If I use the Morden Image Format plugin, do I also need the Shortpixel plugin for compressions simultaneously?

It looks like Shortpixel offers features such as serving images from a CDN, along with some other optimization features. If both work for you, great! There may be an overlap with generating WebP images based on reviewing the Shortpixel plugin, but it’s really up to your personal preference.

Q:2, Is it advisable to use this Modern Image Format plugin to regenerate pre-existing images in AVIF format without any conflicts?

There is no feature within the Modern Image Format plugin at present to regenerate pre-existing images, but there is an open GitHub issue regarding this. There are also existing plugins, possibley even ShortPixel that can perform a regeneration for you, or you can use WP-CLI.