Can i connect via adb to tablet that is stuck on welcome screen and won't boot?
It is possible to connect to the device through adb when it is stuck on welcome screen or even when it is showing boot animation. The problem in your case is that your device has not authorized the connection. Under normal circumstances, when you connect through adb a dialog is shown on the device for user's confirmation to proceed. Until the user agrees, you can't do anything good with adb. And since you're stuck on welcome screen, I see no way to agree to the dialog. 

That been said, if you can somehow manage to flash a compatible custom recovery you can pretty much salvage all of your data. You might also be able to solve this adb unauthorized issue.