Different admin backend
You can create a second AdminSite etc, but that will still be mainly a standard django admin. If you have more special needs, you'd possibly be better developping a custom one (using forms and ModelForms and possibly a couple generic apps for tables etc) - this is all standard django programming... 
wrt/ validations and permissions, they are nothing specific to the admin app.
I'm new with django and I was thinking if it's possible to do a thing.
I know I can access to /admin/ and have django default admin but what if I want to create another admin interface with different URL using the default admin model/widget?
To be clear, new different interface will be for example /customer-backend and people who join it (what about permission?) have another graphical interface but using things (like validation) that are available in django admin backend.
Thank you!