As the comments say there's some stuff in the 'Stay Safe' section on Wikivoyage.
And you're right there doesn't seem to be much other information on the internet, which is odd because I'm sure I found something before I went there a few years back. You may need to pick up a physical travel guide, I don't have mine to hand but I think there was some useful stuff in there.
Younger people will probably speak decent English, although they'll start with Russian unless your an obvious tourist. Anyone tourist facing will likely have some English. In general I never really had a problem without speaking Russian although, as always, it's useful to have to have a few phrases including 'I don't speak Russian'. Although I'll note that the time I did try to say something a little complex in Russian I was met with an uncomprehending stare ...
As for scams, standard common sense is a good idea. False or corrupt policemen are a problem, don't let them take your passport or anything else they can them demand money for. Similarly they may hit you up for a fine if you're caught drunk in the street (or anything else they can think of). I've heard of them taking someones wallet on some pretense or other and then sleight-of-handing money from it before giving it back. Again, I've no idea if these are real policemen looking to make a buck or outright fake scammers.
As well as the normal tourist risks there are more complex scams that usually rely either on the marks good nature or their greed and end up getting some (fake?) police involved. I got hit with a variation of the Money Drop scam in Moscow. But I was carrying next to no money and managed to get out of it just by playing dumb.
You can certainly talk to people in bars, although you may find people aren't as chatty as they are in other countries. Again, common sense, keep an eye on your drink, make sure you know what you're buying and how much it is -- pay per order if possible rather than running a tab, don't get dragged to some other place (or into a car) with people you've just met. Don't tell anyone where you're staying (for the obvious reason and because it gives them a rough idea of how well off you are), similarly be vague about when you're leaving and what you're plans in general. Avoid discussing anything that might be a hot button issue (politics, human rights and discrimination, religion).
Unless you're the sort of person that is regularly approached by attractive women then I would be suspicious if one does approach you. In fact, be wary of anyone that makes a point to approach you and talk to you, particularly if they have a good story.
Although you can certainly manage on your own I would consider getting a local guide for perhaps some of your visit, there are some places that you'll just never find out about without local help.
But I wouldn't be overly worried, St. Petersburg is reasonably safe if you're sensible and (in my opinion) there are much worse cities for scams in Europe.
EDITED TO ADD: And watch out for card skimmers at ATMs, which are reasonably common and people copying your card information in bars / restaurants. Pay cash or keep a good watch on your bills for a few months after.