Can I ? should ?

1st can I use a 600 watt mh in my 600 watt dimmable ballast?
2nd Should I if I can use it in veg to promote nodal growth? 3rd Also I've heard neem oil protects against bud rot I'm thinking on getting some but need to know how much to mix ( metric) and regiment of when to use and when to stop ✋️

Your 600w mh should work fine unless the ballast is ancient, I would veg with mh you do get slightly closer nodes but ime the difference is blown way out of proportion it's only minor.

Idk anything about neem oil I've never used it.

Neem on flowering plants leave a residue that makes smokers sick. If you wanna spray in flower there a few things like lost coast or cann control that you can use.

Air circulation will help against bud rot and won't leave an awful taste like neem.
Better to prevent it in the first place by having best environment.

Mh for veg is great, you will notice the difference on some strains more than others.

Don’t use neem in flower