Hey WTB, I meant to reply the other day but I didn't get around to it.
We've had a few guinea pigs over the years and its taken all of then a few days to settle in. They're very nervous animals by nature so they'll always be a little skittish if they're startled. Usually they man up if there's food involved though!
We keep their dry food topped up and they probably get through a bowl full every couple of days between two of them. They do tend to graze over it, where as rabbits tend to scoff it all down in one go. Feeding them pellets as opposed to the muesli type stuff is meant to be better, as with the muesli they'll just pick out the bits they like so they don't get the full range of vitamins etc.
You should give them plenty of fresh hay to aid their digestion and to play in.
As far as fresh food goes, we tend to give them a bowl full once a day, usually with a handful of spinach in a boredom breaker ball
http://www.thepetshopuk.co.uk/images/34387.jpg We give them a mixture of a few things each day including carrots, broccoli, peppers (red, yellow, orange or green), sprout peelings, cauliflower leaves, cherry tomatoes, cucumber, spring greens, spinach, dandelion leaves, grass, romaine lettuce... Only give them very small amounts of anything "cabbage like" (sprout peelings, cauliflower leaves etc) as it can give them a windy tummy. And only small amounts of cucumber and romaine lettuce as they're very watery and too much can give them the runs.
They absolutely love anything green and will chomp through that first. They can see in colour, so I don't know if its the flavour or the colour they go for the most.
Guinea pigs are like humans in that they have to eat a fair amount of vitamin C to prevent scurvy so fresh food is good for that. I used to put the vitamin drops in the water but they didn't really like the taste and someone (I think Dimrill) told me it can give them kidney stones.
Anyway, they're delightful pets. Once they get used to you they'll start "popcorning" around the cage when they're excited and squeak like mad anytime you rustle a plastic bag because they'll expect to be fed!
Put some pics up when you get a chance