Metallic is often oxidation?
The longer you boil the sugar, the more it will caramalise. I think the candi sugar is invert, probably easy to make, but easier to buy
Edit: Apparently if you add sugar, the yeast will go for the simple sugars first as it's 'lazy', and might not get through as much of the longer chain sugars as it would if the sugar wasn't there. So by adding sugar after the initial bulk of fermentation is done of the sugars from the malt, you get better attenuation. I'm not sure if this is scientifically proven or not.
Add sugars for a long time in the boil and it'll caramalise more so won't be fermentable (like the sugar from crystal malt).
In this case, you're probably better off doing a multi step mash, pitching a good amount of yeast, ramping temp before worrying too much about when you add the sugar.