As with anything that breaks your skin there is a small risk of infection
Please have a read of this link here on way to pick piggies up to take stress out of it for them and you. He is biting because he is scared
How To Pick Up And Weigh Your Guinea Pig
I would advise letting the vet do his nails still as they can obviously do it quicker than you. But if you do want to try maybe o it as a two person job, someone holds piggy and distracts with food while someone clips carefully
We used to have a 'big bitey pig' called Vimto - I used to get actual snaps off Vimto and she would draw blood too
She was just a very scared piggie and felt vulnerable on laps, she didn't appear frightened on my lap as didnt freeze (but obviously was) but she was confident enough with me not just to freeze and it was here way of saying 'please put me home i don't like this' by biting me hard. We worked at it and would take her out for 30 seconds then pop here home, we did this 3 times a week for around a month then slowly increased it up to 5 mins via 30 second increments . Each lap session she'd have a sprig of coriander. It took us 6-8 months before she stopped biting and she would happily chill on laps for a while but not for too long
We would never let anyone else have Vimto on there lap in case she wanted to taste the forbidden flesh once more