To be sure your site work on most mobile phones I recommend you sign up at Nokia Developers and download the free Nokia S40 and S60 emulator - They are rather big and demand you also have Java installed on your PC - But if it works on Nokia S40 and S60 emulator you cover an audience larger than most and can see if it works on WAP and earlier versions of xhtml mobile browsers.
Then after that you can download various files - open them on the emulator to test mobile browsers like skyfire, Bolt, Opera, UCweb, etc. But REMEMBER the browser will take most websites and make them look good even without the plugin!!! So there is no reason to use the Opera Mini for testing the mobile plugin because IT WILL work with or without