Hi, what do you guys do for slow motion footages ?

5D3 don't have 1920x1080 50fps or 60fps . It has 1280x720 50fps only.

Can we take in that resolution, slow own with Adobe PP and join to our 1080p movie ?

I use 720p 50fps footage on a 1080p 25fps timeline when I need slowmo, and it looks really OK. Just try it and see what you think. BTW Mine is shot with an XF300. I have a 1DMK4 with similar video specs to your 5D regards resolution and frame rates, but have never thought to try it with slow motion. I'll try and put that to rights tomorrow.

Yes, use 720p60 for slow motion. Shutter speed 1/250 and you'll be good. If you're final project is 1080p then you'll have to up-convert the footage, but at slow motion, it's ok.