Just to finalize this thread, for those who have this question in the future.
I had around 1400 posts already on my site, and none of them had tags. My site is autoblogging every day updating with new television episodes. So there are a lot of posts for the same episode. Example: there are 27 different epsidoes posted for "House MD".
I tried the two plugins mentioned above: tagpig and strictly auto tags.
I ended up using "simple tags" which is free, and you can find it by searching the wordpress plugins site. It supported OpenCalais, Alchemy, and Zemanta. I assume these are sites that the plugin will look to for tags on your new posts. I think TagPig used these same sites (or similar), but you have to pay for the Pro version of TagPig. This Simple Tags plugin supported them and is free

. So imo this is better than tagpig, because it is free and does everything it can do and more!
So my next concern was, I have 0 tags loaded into my wordpress settings. I manually went through all the posts on my site, and made a list of the episodes. Then added a comma after each one and made a list..
Example: being erica,hells kitchen,lost girl,dexter,the league,the mentalist,warehouse 13,eureka,boardwalk empire,merlin
Using simple tags, go to "Tools > Auto Terms". I have "activation" and "Target" checked. Then in the keyword list, paste in your entire list.. "being erica,hells kitchen,lost girl,dexter,....."
This keyword list is what will mass auto create your wordpress tags, and also add the tags to the matching existing blog posts.
I went from 0 tags, to 150 tags effortlessly. Now all my posts of episodes, have the matching tag.
Mission Complete!