We can classify twitter users a number of ways. We could look at their purpose on twitter: Is it a personal account for chatter, a business account for engaging customers or a hybrid account? I think most authors will be hybrids - we do use twitter as a platform to show off our creative works, but it's a highly socialised form of selling for most.
We could further separate users by demographic which might be handy in targeting those who are most likely to own a kindle/ read/ read our genre etc. This is pretty labour intensive, and as it's done on a statistical likelihood it's not hugely accurate. My gut is that anyone who opts in to following you in the full knowledge of your genre is probably part of the demographic whether they fit the trends or not. The only time you'd use demographic targeting is if you are attempting to gain followers using a follow/ unfollow method (Which is essentially follow a thousand people per day. If they don't follow you back you assume they aren't interested and unfollow them. Then you follow new people. Eventually you'll have more followers than you follow. I don't recommend this as you aren't engaging with these people - but it does give you raw numbers, and it worked for John Locke).
My own preference for dividing users up are those who want to engage versus those who want to follow/ be followed. The latter sheep/leader types usually follow far more than they follow. If you are being followed by a huge number chances are you are putting out interesting content (or you are silent so no one notices). Unfollows occur when you spam, or when you put out content inappropriate to your existing following. That doesn't mean you've written bad content, merely that those who had opted in to reading your tweets didn't like it. The prime reasons for this in the literary twittersphere are spam advertising & shop talk between authors putting readers off. Readers don't care about the latest changes to the Amazon algorithms, the upcoming launch of PubIt or other self publishing chatter. They might like your sales stats, how much you've written today or similar but that's because they are interested in you as an author (and probably why they continue to follow you). We can all tolerate a little shop talk, but if we constantly see a conversation with in jokes we will just stop listening. Readers are the same.
It's why some of the big names have moved to using two twitter accounts - one for readers, and the other writers. I'm not sure I could keep up two accounts, but in an ideal world all the shop talk would be hived off away from prospective readers (this post gains me no reader exposure whatsoever - it's a dead loss in terms of marketing). It does help build the indie community which is great but it doesn't put food on the table.
I'm a strong advocate for only engaging with those you find interesting - I don't follow just because people follow me. I follow those I find interesting. Mention spam and the like are offputting, and serve no purpose in me reading it. I don't want to buy your book nor do I want you to spam my readers. I'm happy to RT if you ask me nicely (or join the GRTweet, WLCAuthor or other RT groups I participate in) but every tweet that isn't 'useful content' i.e. amusing or informational lowers my signal:noise ratio, and thus dilutes my brand. Look at the big name celebs - and then compare their tweet rate to your own. Three or four genuinely interesting tweets a day gets you a lot more followers than the 50 plus tweets typical of most authors. I know I'm guilty here - I used twitter to chat and procrastinate which is fine because it's my own dime I do it on, but it doesn't help promote my books, and so I probably shouldn't do it (I will freely admit my own hypocrisy here - knowing what's best and doing it are two entirely different propositions!).
So, follow those you like, don't follow those you don't and put out the best content you possibly can. Amuse, inform and provoke debate - anything that engages will probably do you some good. At the end of the day though do what you want to do - and live by the consequences. If you want to automate your tweets to spam people, that's fine but it's worth keeping an eye on your unfollow rate via who.unfollowed.me or similar to see if it is actually working for your core following. If not, it might be time to either find a new following or change the way you engage with twitter.
Oh, and those robot twitter accounts do no harm - unless you follow them in which case they will DM spam you (which is a great reason not to auto follow back). Twitter periodically deletes them which is very handy, so just ignore them and carry on chatting.
See you in the twitterverse.