When you save your form, you can choose static pdf, or dynamic pdf, and if you save as xml form, you seem to get this error. Try saving as not xml form. However, if you are using scripts, you must save as dynamic form.
The only way you can combine it is through Assembler and the form must be the 1st document in the set. You cannot combine multiple forms. Conversely you can flatten the form (on the server - takes the fields out of the form - removes the interactivity from it). Then you can combine it in whatever way you want.