So install Windows XP, pull it out and put it in a Windows 7 based system as a secondary drive and then shrink the volume there. Is that right? And how much do I shrink? 20GB?
No. You can do that in Windows XP too. Shrink it however much you want. 10-12GB OP free space is fine.
Also, once you install XP you will have to align the partitions. Use this, it is easy and free: http://www.partitionwizard.com/partitionmanager/how-to-align-ssd-partition.html