Am i able to install Win 10 on a Macbook Pro '15 running Catalina (model no. 12,5) with/without bootcamp properly?
If you wish to create a USB Windows 10 installer, the process if fairly simple. 


Use the Boot Camp Assistant to download the Windows Support Software (the drivers).
Download the Windows 10 ISO from this website.
Erase a 16 GB or larger flash drive. Use the Master Boot Record scheme and ExFAT format.
Copy the files from the Windows Support Software download and Windows 10 ISO to the flash drive.


If you do not wish to use the Boot Camp Assistant to install Windows 10 to the internal drive, then apply the following steps.


If you are going to keep some existing partitions, then create free space or contiguous partitions which can be deleted during the Windows installation.
Restart the Mac and hold down the option key until the Mac Startup Manager icons appear.
Select the EFI Boot under the external drive icon.
When asked for a Product Key, you can select to skip this step. If you skip, then at some point (30 days?) you will need to enter a product key. 
During the installation, either erase the entire internal drive or install to available free space. This may require first deleting any unneeded partitions.