Since I plan on putting talents into Alchemy and maybe signs, I'll have to carefully choose what to put into combat styles. What I'm wondering is what's the best for each blade. For example, I'd guess strong type would be good for silver, since I'd imagine a lot of monsters would be fairly large.

Also, is group better than speed or vice versa? Is it better to go with speed and take down groups of weaker opponents one by one, or go with group and be less effective against lone weaker guys?

I think all styles are important, and you need to put some effort in advancing all of them. At least distribute talents to those spots which maximize damage for those styles (that would be the bare minimum IMO).

You have plenty of slow and fast enemies suited to each type of sword and sometimes the large groups are a mix of the two, so there is no "best" style. You might be able to get away with group combat only, it works against slow and fast enemies though not as well as strong and fast styles. It doesn't work too well, if at all, against a single enemy so that is where you could use magic I suppose.

I wouldn't worry too much about it however, I went the opposite way of you and boosted weapon skills instead of magic. I have some pretty powerful magic as I have run out of places to spend my points. If anything make sure you boost your attributes as they effect multiple skill trees.