As IAAL said, we need a lot more information to be able to try to help....
However, I will give you some GENERAL answers to your questions:
Q1) What is the law on IOU'S.
A1) Too broad. What are you asking?
Q2) Does the holder of an IOU have absolute rights to the property promised by the signer?
A2) Maybe. It depends on a lot of things. Does the debtor have absolute ownership, or are their liens on the property?
What kind of property? Also, depends on the wording of the IOU.
Q3) What is the law on multiple IOU'S floating around, that is, the plaintiff has one IOU and the defendant crafts a dummy IOU to thwart the original.
A3) Again, way to vague. What are you really asking?? Does the debtor issue the 'multiple' IOU's, or are their 'offsetting' IOU's???
Q4) Wouldn't the plaintiff have to sign off the 1st IOU to void it.
A4) Again, too vague. Are you meaning does the writer of the IOU have to sign it?? Or, does the holder have to sign off the IOU as paid to 'invalidate' it??
Q5) Can forgery charges be brought against a defendant in a small claims court?
A5) No. Forgery charges are a criminal action. However, you might be able to claim fraud and recover your damages.
Q6) Is there a generally accepted time frame for IOU'S to be paid back? they are short-term < 1 yr? yes?
A6) No. There is no statutory time limit for IOU's, just like there are no statutory time limits for other written contracts. Each contract (IOU) stands on its own. If there is no specific time for repayment, then it is open ended and without a payment term.