WP Spam Hitman allows you to fight with spam using set of patterns and hitpoints. It also allows you to decide what's going to happen to the spam
You simply write rules in WP Spam Hitman options in Rules section. Every rule in seperate line. You can use words of regular expressions
No - if your rule is for instance 'p0rn' it gets hitpoint every time Spam Hitman finds it in comment. So if the comment if 'p0rn p0rn and one more p0rn' - it gets 3 hitpoints.
By default - every rule is a word. But if you start (and of course end) a rule with / or # character - it will be treated as regular expression. Small hint here - word rules work faster - so if you can write a rule as word(s) - do so.
No - words are case-insensitive so 'p0rn' will find 'p0rn' as well as 'P0rN'. On the other hand - regular expressions are case sensitive - so you have to use regex modifier (/i) in rule to match both - upper and lower cases.
Go to WP Spam Hitman options and delete the message. If message is blank - WP Spam Hitman will not display it to spammer.
There are no "the best rules" - it depends on blog. For instance I have my blog written in Polish (well - mostly) so for instance some popular words in spanish - they probably spam. There are some trends, some popular spammer words/expressions. Most of your visitors will not tell you to 'enlarge your penis' and provide you with a link - so the rules could be 'enlarge' and 'penis'. If you're not sure your rules works - use options moderation for some time and check if all of the messages it caches are spam.
The short anwser is - yes. WP Spam Hitman should work well with other plugins - not only with the spam ones. On my blog I use Akismet, WP Gateway and WP Spam Admin - and they work just fine. If you find one which doesn't work - be sure to tell me about it and I will try to fix it. But please remember that WP Spam Hitman has high priority so if it will find spam and for instance - delete the comment - it will probably not go to other plugins!