If you don't mind walking to th eunit to change the settings, then ANY flashgun set on M or old fashioned A will fire with an optical slave - so the SB26 will work, it just cannot be controlled from the camera and will only do what you tell it.
To prevent the "not bought yet" flash from firing when the signal flash goes off, you can either use FV lock fiddled in the custom menus to the preview button, OR get all the flashes going in manual. The CLS uses a series of pre-flashes to sort themselves out, any non CLS unit will fire when it sees these flashes instead of the main, taking flash.
For simplicity, I would think the extra £60 is worth every penny. If the occassion demands it, you can set all the remote flashes to M from the camera anyway, or any of the other modes. At least you would have compatible units, not one odd man out.