all standard 32-bit applications do run w/o problems under Win 7/8 x64, applications which are using software copy protection mechansims as e.g. writing a token to invisible areas of the HDD (AC: C-Dilla) or applications which are using undocumented/unsupported methods for accessing the OpenGL stack may obviously fail. 16-bit apllications are of course not support with 64-Bit Windows 7/8 versions any more.
Because Windows 10 is an evolution of Windows 8.x only, it’s very unlikely that standard applications running under Windows 8 will be rendered unusable by Windows 10.
For running mutiple operating systems on one system several drives (or partitioning a drive) is not required, a virtualization software as e.g. Parallels Desktop or VMware or the free VirtualBox (or XP mode of Win 7 Pro+) allows to run an additional operating system virtualized under the parent operating system.