The multiple speed units on the gasifier units are used to fine tune the input air flow based on conditions. A lot of commercial units don't allow for the best range of adjustment. Primary:secondary airflow in a gasifier is 1:5 respectively and depending on design constraints, the fan speed is changed to achieve a specific heat output value. Although I feel personally that this method is more of a cost savings issue more than anything. Why pay big money for a mass run of custom sized fans when you can take something cheaply available and adjust it till it works?
Read the fourth and fifth posts, they explain fan sizing and pertain to both gasifier and regular wood burners. The burning is the same in both types, its the control we have that differs.
http://outdoorwoodfurnaceinfo.com/forum/index.php?topic=1182.0Determine your heat load needs first, then you can design everything from there. If you have any other questions feel free to ask, we are all happy to help.