Inductor 100uH wrong size

6x8 doesn't have the current handling capacity. They can overheat quickly if you are using bright or large tubes.

If you can't use them in any other way, they are good for buck converters dropping 12V to 5V using an MC34063.

I don't think that is a good idea. Unless you can find the current rating for the part and you are not sure it is >=1A, I think it will fail quickly.

If you want to try anyway, leave the leads on the inductor quite long, so it will be easy to substitute. If the part is high quality, it might be OK.