Do you need to uninstall all adobe programs if you're switching to photoshop only plan?

I would maintenance vigilance on the charges and not take a chance. This is only from personal experience. You may have to pay through the end of the period. You can contact Adobe subscription. They have been preety good if you notify them immediately. 

I know that if I were completely dropping the Adobe plan, then I would uninstall all programs first.

By @RealAnise

Actually it doesn’t matter what order you do it in, because if you left them installed and cancelled the plan, the next time you start any of the still-installed programs it would just say it can’t run because no plan is active, and then you can uninstall it whenever you want, or leave it installed and later restart a plan. One reason it doesn’t matter is that this makes it possible to install on several computers and only activate the 2 you’re going to use at any given time, freely deactivating and activating computers without uninstalling anything (since a single-user license allows two activations used by you).

If you are going to a single app plan, you can leave Photoshop installed and use the Creative Cloud desktop app to uninstall anything else.

Because you said “Photoshop only plan,” there’s one odd option to mention in case you don’t already know…

The Photoshop single-app plan is USD$22.99 a month, but the Photography (20GB) plan is a special bundle that’s Photoshop, Lightroom Classic, and Lightroom for USD$9.99 a month. For many people, it’s better to do the Photography bundle even if you don’t install all of the apps. So if you wanted to, you could get the lower cost Photography plan and only use Photoshop.

If you have the standard one year license, I assume it will run the full year. You probably need to cancel your existing license first, and then when the term is out, you sign up for a new license.

There's no limit on what you can have installed, but they are activated according to your license, through the CC desktop app.

Try the chat here https://helpx.adobe.com/support.html?src=community . As I understand, typing "agent" will put you through to a live person (someone correct me if that's wrong).