Thank you for the post.
"Can I continue to use FMS12 but use FM13 clients?"
Yes, the two product lines are compatible.
The FileMaker products that use the ".fmp12" file extension are able to communicate. FileMaker Pro 13 can access files hosted by a FileMaker Server 12, and FileMaker Pro 12 can access files hosted by FileMaker Server 13. The same applies to the FileMaker Go product line.
Please let me know if any of the above needs further clarification.
We experiment some issue with FM13 (ADV - not tested with Pro) that open database on FMS12 :
When we insert an image on a layout (from menu "Insert > Picture") and resize this picture, the image disapear and is replaced by a "Missing picture.png"
On FM12, render is ok
The red box around the picture are a border to see the size...