The model difference is fundamental: Smush, ShortPixel or Imagify compress and/or convert the original file at upload time, sometimes overwriting it, sometimes generating a WebP copy alongside it. ImagePipe never modifies the original file: it generates its own variants on demand, in its own disk cache, separate from the media library.
The risk if both run in parallel: ImagePipe then receives a source file already lossily compressed by the old plugin, and compresses it again in turn. Result: visual quality degrades without a meaningful further weight reduction, and sometimes worse, JPEG artifacts stacking up from one compression pass to the next.
Steps for a clean handover: disable both automatic compression on upload AND WebP/AVIF generation in the old plugin, both, not just one. You can leave the old plugin active only for a feature unrelated to images (a static-file CDN, for instance), provided you check case by case that it doesn't conflict with ImagePipe's automatic replacement.
Files already processed before the migration cause no problem afterward: since ImagePipe ignores the weight and format of the stored file and always regenerates its own variant on the first real visit, a history of files already compressed by the old plugin needs no cleanup or re-upload before switching over.