I've used both. 7mm is fine for oly, but I'd only recommend if your knees are totally trashed. 5mm is enough if you're knees are fine and just looking to keep things working well.
Both are IWF compliant.
>should I train my pulls before or after squatting?
Either is ok, but I would do it before more often. Doing anything after back squats will force you to work in a fatigued state. You'll learn a lot about your technique but it's not something I'd do a lot.
Could be. But what's more likely is your technique is holding you back. Thats usually why people are behind on ratios
