(1) For any OS to access a partition is to mount it. If you make the personal partition a NTFS type then Xp will mount it automatically so half the problem is solved. In Linux most distros will mount a NTFS partition too.

(2) Once mounted Xp will automatically give it a drive letter. It will be D no matter where you put it if you have only one disk. Mounting Linux partitions requires 3rd party drivers and it is optional.

(3) Generally the system-related software of one OS has no use to another and there is little point to mount it. You can use either NTFS or EXt2/3/4 but the easiest is NTFS.

(4) Since Xp will mount the data partition so it will be your D drive (assuming no other disk). Other Linux partitions require you to install a driver and mount them by specifying your own drive letters which can be non consecutive. Thus the position of the personal data partition has no consequence. However if you put it at the end you can control its size better (by allocating a smaller size initially and later by resizing it by absorbing the unallocated space.

Resizing an OS partition will cause problem because every boot loader boots an OS by its hard disk address embedded during installation. If you resize an OS partition, other than sda1, you will have to repair the boot loader afterwards. Thus keeping the data partition at the back of the hard disk helps.

(5) Xp only supports partitions it recognises. Those partitions can be made by other OS. That is the whole idea of able to use different OS. Fat partitions do not have Ownership and are easier to get damaged by attacks.

(6) MS Windows do not entertain booting from an external disk vis USB2/3 (but OK if it is eSata) . Not all Linux can be booted from an external hard disk as the system files will have to be rigged for such purpose.

If you install Xp to the new hard disk but using the same license as the existing disk (regardless if it is a Master or Slave) then Xp will not like it and will do something to the other copy making it unbootable. It is advisable to remove the old Xp and link it only as a USB disk.