Hi Cassidy,
It's really a matter of personal choice to use your ring or middle finger on the "fundamental bass." I generally use my ring finger and this is considered the "modern" way. It actually makes it easier to add 7th chords in my opinion. Many experienced people end up using both anyway. I recommend you start learning using the ring finger.
If you are looking for the Sedlon course, you can find it, with audio lessons free here:
Scroll way down till you come to the chart of lessons in zip format. This is how I started.
I think Dale Mathis's lessons have already been suggested to you:
I understand it can get boring to play all polkas all the time! I like to play some nut mix it up with lots of other music. Don't worry about what you learn to start with. It's just to get going, then you can pick what suites you.
My favorite Cleveland (area) band is Ted Lange on accordion and Mollie Busta on trumpet, vocals and other instruments generally:
Good luck!!!!!