Carlos, the idea of converting plans from plywood to aluminum initially appears to be a simple prospect of apply a uniform conversion formula- but that is not the case. The two materials have different properties so the framing and hull material needs are not exactly 'convertible'.
I would suggest that you take the plywood plans as a beginning, then using Dave Gerr's Book "The Elements of Boat Strength: For Builders, Designers, and Owners" http://www.amazon.com/Elements-Boat-Str ... 0070231591
work out the needed framing and scantlings required for that particular boat.
I would say that in smaller boats, under 25' this book will result in a heavier than absolutely needed skiff, so instead of rounding the calculations' results UP to the next larger or thicker material as Dave suggests in his book, I'd round down.
As an example, the formulas in this book may recommend a bottom that is 4.5 or 4.6 mm, but I'd say a 4 mm bottom for an 18-20 foot skiff was plenty of material- and there is no need to round up to a 5mm bottom panel. However, if you follow the book's formulas I'm positive you will be able to create and adequate frame with thick enough hull panels to convert a plywood design to aluminum.
The book may recommend that framing material was 6 or 7 mm thick and again I think 5mm would be more than adequate for both ring frames and longitudinals.
Its important to remember that the book was intended as a guide, not a boat by boat designer's conversion reference. So the book works well to give a fine set of hull strength details while also allowing you to plan frame spacing, stringer spacing, intersection details and other critical aluminum building suggestions.
In the end you'd use the plywood plans to give you all the shapes, lines, layout and arrangements, but covert the framing and hull panels to aluminum using the formulas Dave Gerr provides in this reference book.