The printers have to be TWAIN compliant to be supported. If the printers you have are not, then I doubt those will ever be supported.
What TWAIN compliant means -->
https://answers.yahoo.com/question/index?qid=20060720073922AAHzQ1TList of TWAIN compliant drivers -->
http://www.inspectortwain.com/default.aspxThere are scanners not on the above list that supposedly TWAIN compatible drivers and yet still seem to refuse to work with GTS (such as my CanoScan 5600F)
To confirm that your TWAIN driver is working, try scanning some images using InfranView
http://www.irfanview.com/Quick Check Procedures:
Install and launch the program
Go to File -> Select Scan/TWAIN source and see if your scanner is listed.
Then select the next option below "Acquire/Batch Scanning" and click OK on the dialogue that pops up. You should be able to scan your image
Hopefully you'll be able to get your hands on a scanner that is compatible with GTS.