It is super normal. A lot of Guinea pigs are very vocal, and they have a wide variety of sounds they make depending on if they're happy or sad.

Sometimes they mistake you for food if you cut up some veggies, things like that, but in general, it means that they don't like whatever you're doing, so stop.

They can bite. Hopefully, if you socialize them well enough, they won't, but if you startle them or they're not well socialized and not used to something, they can bite.

They can be. Many guinea pigs are friendly and enjoy scratches, but it depends on how well socialized they are.

It can be tough if they have darker nails, but having little trimmers may help. If they're white nails, you want to take the white part and not touch the pink part at all. If they are darker, then you start with tiny bits to make sure that you're not going to hit their skin and make them bleed. If you're uncomfortable or don't have someone to help you hold them, we can do it for you. Otherwise, I think some of the local humane societies have $5 events every couple of weekends, where you can bring in your guinea pig, and they'll trim the nails for you.