The Gmail app will only accept google mail accounts, whereas the email app will accept any account including google mail accounts. For instance, I have an Orange email account, a hotmail account, an account associated with my website, all of which have been added to my email app, I could also add my gmail account/s but I am quite happy to keep them separate.
You could do one of two things :-
1) Add all your email accounts, including any google accounts, to your email app and don't use the Gmail app.
2) Utilise the function that enables a google account to 'fetch' emails from up to 5 other email accounts. If you have more than 5 other accounts simply create another google account. Add your google accounts to the Gmail app and don't use the email app. To set up the 'fetch' function you need to go into your google mail account settings using a laptop/PC. Go into Accounts and you will see 'Check mail from other accounts'. Setting it up is fairly straightforward.
As far as uninstalling apps is concerned, generally speaking any apps that are pre-installed by the manufacturer/carrier cannot be uninstalled, any that you add from play store or other sources can be uninstalled by going to your apps list in Settings. Tapping any app in the list will open an action page and there will be an Uninstall button where appropriate.
Hope this helps