Can SAP GUI scripting be used in conjunction with GuiXT scripts?

Yes, SAP GUI scripting can be used in conjunction with GuiXT scripts. The only restriction, so far as I can see from having tested the procedure, is the following:

If the GuiXT script deletes a certain element (entry field, button,..), it is no longer known in SAP GUI scripting.

Otherwise, GuiXT layout changes do not affect SAP GUI scripting. If new screen elements are added with GuiXT (pushbuttons, radiobuttons, or - with GuiXT InputAssistant- new entry fields), they too can be automated with SAP GUI scripting.