are these forms my best or a feasible option- wordpress, zendesk, s2member

I'm sorry: we don't offer any telephone support.

Can you let us know what information you want to collect in your form, and what you're going to do with it after you collect it? We'll be able to help you better with that information. Thank you.

OK, first I'd like to have a user registration form for my WP site that includes- email, password, username, cc # (possibly), a FB register/login (if possible) and a submit button.

You can collect email and username for sure. If you collect a credit card number, you will need to follow the PCI guidelines for storing that information https://www.pcisecuritystandards.org/ (and have an SSL certificate installed on your server.) Facebook registration and login are not features of Gravity Forms.

BUT, after hitting submit to register in your form on my WP site, I'd like the user to be presented with an area in the same place as where the WP reg form used to be that contains a subject field and message field.... priority drop down menu if possible.

Sounds like you would like to direct the user to another form, after they register, where they will submit a message to you, with a subject, body and drop down selection. If that is the case, you can certainly direct the visitor to this new form after they register using the first form.

I want this info to push over to zendesk.

There is no add-on I am aware of for Zendesk. However, there is talk of a Zapier integration: https://zapier.com/zapbook/gravity-forms/zendesk/

And, in the absence of any official add-on, you can always send data to any 3rd party you like by using the gform_after_submission hook. Documentation for the hook can be found here: http://www.gravityhelp.com/documentation/page/Gform_after_submission

Then finally I'd like on that first reg form I spoke of above, to have the option to select 'login' for already registered members.

The way I would handle this is offer a login link in the form description or above the form, in the WordPress page. That way, they can log in first rather than attempting to register and getting an error about the email address already being registered.

It all made sense. Let us know if you have any more questions.

Gravity Forms need to be embedded somewhere. You can embed it in a widget, or a page or post, or a lightbox. Where you embed it is up to you, and the way it looks is dependent on the template or location you choose to display it in.