Q&A for How to Configure a Computer to Multi Boot Operating Systems

Yes you can, you just need to have two partitions for each OS. For Instance, If you have 1TB Hard Drive and want to install Ubuntu. You Could give Windows 500GB and Ubuntu 498GB and 2GB for swap. You can do whatever you want.

Most likely not as Windows XP computers are over 10 years old and Windows 10 most likely will not work on a computer made before 2011.

Yes. But if you want the GRUB menu to appear at every startup, you will have to set the SSD as the initial boot device. Otherwise, you will have to press a key to select the SSD as the boot device if you ever want to boot Linux.

You can create a partition with a file system that both Linux and Windows can read and write, if that's what you're asking. Linux can read and write to FAT32, NTFS, and exFAT. There is a third-party driver for Windows to read Btrfs file systems.

This is probably not possible. To dual-boot Windows XP and Windows 10, your computer must have a legacy BIOS or a Compatibility Support Module. You will need to convert the drive to MBR and/or reinstall Windows 10. Then, install Windows XP, boot from the Windows 10 disc again, and run the Startup repair.