You can install first the Solaris for x86/x64 and then Windows. Do not install Solaris SPARC as this architecture is not supported by Windows OS.
GRUB on x86/64 supports both UFS and NTFS File Systems and will let you choose the OS to boot according to your preference. I haven't tried it but I think it will work.
On x86 systems it is possible to create a fdisk partition of type MS-DOS (Windows OS).