I have always understood that the only safe branches to give piggies are from apple trees, however, I will be interested to hear what anyone else says.
As far as acorns go, I have a very old (1920's) pets book, which says you can feed guinea pigs acorns, however, it also says a lot of other rather odd things, which I doubt would be recommended nowadays. Apart from in this particular book, I've never heard of letting them have acorns, and I don't think I'd risk it. If in any doubt at all, play safe, that's my motto. There are plenty of other things with which to feed guineas, or to give them to play with, which we know for sure won't do them any harm. Having said that, my piggies often crunched up dried leaves that they found in their run, because I wasn't quick enough to remove them before the piggies spotted them, and they were okay! But I just prefer to be very cautious!
I remember seeing once on a wild foods programme, that humans can eat acorns if they are treated to remove the tannin which gives the acorns a very bitter taste otherwise. Might be useful to remember in these days of rising food prices!