I've never had a problem with bullets 123 gr and higher, that said I would imagine you should be okay down near the 100gr mark. If you get two holes from one shot on paper you'll know I was wrong. ?
All depends on use, a few factory rifles are offering a 1:7 twist. 1:7 should work great for heavies and all copper. Nothing wrong with 1:8 its the most popular.....