AFAIK Agama does support legacy (i.e. non-UEFI) boot. It is announced to be deprecated in some (not yet specified) future version, but currently you still can install it on legacy boot.
See also Legacy BIOS boot opensuse leap 16 and agama storage - #3 by naive2021
This is decided on the BIOS level; either your BIOS is set up for UEFI boot, or it’s set up for legacy boot. This is nothing that can be changed on the OS installer level.
See also the Leap 16.0 release notes and the Agama documentation.
One caveat: Some advanced features like full disk encryption are only supported in an UEFI scenario. But since you plan to keep your old Windows, this doesn’t apply in your case anyway; FDE means wiping the whole disk.