The E-Commerce extension's scope it fixed to All Tags because the intended use is to provide one location in which you can map all of your E-Commerce data and have it automatically go to your Tag vendors. In this case you're sending exactly the same data to multiple vendors. You can always override the E-Commerce extension for a particular Tag by mapping to its E-Commerce destinations directly.
If you need to send different data to each of your Tags, then the E-Commerce Extension isn't what you'll want to use. In this case, you will want to map to the E-Commerce destinations for your Tags manually, so you can specify exactly what data goes where.
You mentioned that you only want 'it' fired on one page. Do you mean you want Google Analytics loading only on one page? If so, you can create a load rule (https://community.tealiumiq.com/posts/572034-load-rules-creation)
to make sure GA only loads on that page.
If you mean you only want the E-Commerce extension to load on one page, that is not something you can do through the extension itself. Let's remove the E-Commerce extension from this situation. In this case I assume you only want to send E-commerce data from that one page to GA and you don't want to send E-Commerce data from any other page. For this I would recommend using a Set Data Values extension (https://community.tealiumiq.com/posts/554049-set-data-values-extension).
With this extension I might try to:
1. Blank out the E-Commerce data sources (perhaps set them to empty strings or arrays, depending on the data type).
2. Set its scope to Google Analytics so it won't affect other Tags.
3. Set up its conditional logic to make sure that it won't blank out the data sources on the page that you do want to capture data for.
For your last question, I'm afraid I don't have enough information to really help you. I recommend that you contact your Tealium Account Manager to investigate the details of your implementation.
I hope this answers your questions.