Glad they are settling. 
Some piggies do give a nip (if they wanted to bite you , believe me they would do more than nip, those rodenty teeth are sharp) but they are by and large gentle creatures. Don't forget though, they don't have much in the way of defences, so if they feel unsafe ,they panic.
Our GP2 got my arm (like you say a pinch not a bite) when we had them for a couple of days and I was 
"He doesn't like me" . But usually, any tooth action is if they mistake the smell of food on your skin.
GP1 only gnaws if we rub his rump (greasespot area)
GP3 doesn't like his right side touched , he pulls our skin rather than bite, but it's a definite don't touch. (Dh didn't know this. He does now)
Water- mine never drink. I give them a bottle if they are indoors and a bowl outside but they don't touch it. They pee rivers , it must be the veg. 
I don't think you can overfeed but some foods are more calorific.
They need unlimited hay, and about 1-2oz pellet per pig ( you'll gauge how much they'll eat but make sure there's a bit spare so there's no squabbling over food)
What coat types are your boys? I find the fancy coats (like Abbys or long haired) tend to be lighter.
GP1 is a smooth haired and lost alot of weight when his brother died.
GP3 is a Rex, he's huge , solid, heavy but not porky.
When we try to get weight on GP1 - sweetcorn ,especially in the leaves. Raw beetroot . Banana and skin. Sweet red peppers.
So if we wanted to slim them, we'd cut these down.
And they can get surprisingly tubby on grass (just like little fat ponies do in a field )