1) yes, you should have your own sts or use an existing one. Writing one is difficult if you do it for the first time but easy every other time then. I wrote a detailed tutorial once, it is for wif 4 but should be easy to port to wif 4.5.
http://www.wiktorzychla.com/2011/08/quest-for-customizing-adfs-sign-in-web.html
2) yes, you can use forms authentication for both sts and your application, however, since forms has numerous subtle issues, it is recommended to use SessionAuthenticationModule to maintain user sessions.
http://www.wiktorzychla.com/2011/08/quest-for-customizing-adfs-sign-ing-web.html
http://www.wiktorzychla.com/2012/09/forms-authentication-revisited.html
Switching to SAM doesn't usually involve other changes in your application if you rely on common authentication and authorization mechanisms (web forms authorization, mvc authorization filters).