Aha, that bites that they used a 1.0mm oring in the design. This is one of things makers do for some reason (I think maybe the sleek look, I dunno?) that really makes it tough to end up with a glass compatible device. There just isnt enough rubber there to cover the tolerance range of glass. Now if the device is designed for the express intention of not using glass then I can understand it and of course it would be up to the customer to figure out how to retrofit if they want a glass tank but honestly i think most folks feel when they buy a device it should work for whatever they want to use on it.
On the oring sizes, I see now why such a small diameter 10.6 x 1.6mm worked out for you because it was really stretched to get on there where a 18mm originally sat. That would be 18 or 18.5mm btw, I know they can be tough to measure (orings are ID x CS - Cross Section) since they move around easily but you get close then go by the standard sizes to figure out the rest.
Not a chance on the 1.3mm CS, I think there might be some Japanese Standard like 1.25mm but for the most part it goes 1.0, 1.5, 1.6, 2.0mm for widely available sizes and these are the only sizes we have. Another reason why its nice when 1.5 is used because 1.6 being just a touch thicker can often be used as an adjuster.
Google 1.25 orings and you can find them around. I would guess you are right in looking for something like the 16-18mm though I dont really know how the fit is and how the different oring CS will settle in the groove. Thats something you just have to play with but the 1.25mm will certainly give you just a little more material and sit into a groove designed for a 1.0mm much better than the larger CS ones will.