I think you are correct in that isinglass and chitosan are interchangable.
Electrostatic Charge and all that jazz
Our three basic fining aids; copper finings, auxiliary finings and isinglass finings are all charge-based processes. All molecules in solution or colloidal suspension possess an electrostatic charge. The amount of charge is specific to the substance in question and is highly dependant upon the pH of the solution it finds itself in, but we will come to pH effects later. An electrostatic charge behaves just like a magnet, except that a magnet has two poles, whereas a molecule has just one pole or a charge of just one polarity, either a positive charge or a negative charge (or a neutral charge). Protein, for example, has a positive charge whereas yeast has a negative charge. Going back to our magnet analogy, we know that opposite poles on a magnet attract each other, whereas like poles repel. The same is true of an electrostatic charge. Molecules of the same charge repel one another and force each other apart, whereas two molecules of opposite charge will attract each other and coalesce. Protein molecules have a positive charge and when in suspension these molecules repel one another which spaces them away from each other, and they cannot easily come together to coalesce into larger particles heavy enough to drop out of solution. Yeast behaves in the same way, except that it has a negative charge. Finings have the opposite charge to the stuff we are trying to remove. The finings then attract the yeast or protein to itself, causing larger, heavier flocs which drop out of suspension faster. Isinglass finings, for example, has a strong positive charge. When isinglass is added to beer its strong positive charge causes its molecules to space themselves out like a web and attract negatively charged yeast cells to it. This then drop down though the beer, attracting more yeast cells to it, which are pulled out of suspension with the isinglass.