If I pre-beat grid my music in ableton Will engine prime mess with the grid when I analyze it or will it retain the grid.
I believe all other beatgrids that aren’t engine will be overwritten with the primes
Engine Prime will make its own grid but it shouldn’t be any different to your ableton ones.
You may need to adjust slightly if it’s the off by a bit.
Everything you do in Prime (including setting grids) is firewalled. There is no way you can “delete cues/bpm/grid/loops” if you change them in Prime, regardless of the audio/DJ software used.
In short you’re safe 
The grid (and any other info) for Ableton is stored in it’s own seperate .asd files which must be saved together with the audio file.
You pre-grid your music in Ableton… Do you render it to a .wav / .flac / .mp3 after? Because Ableton does not add info like that on a music file if you do adjustments to it, keeping/saving it in a project file, without rendering that result to a new .wav / .flac / .mp3…
If that’s been done, Engine will give it a beat grid that’s most likely fitted to the tracks. (sometimes some minor shifting to place perfect need to be done but that’s easy to be done)