That filter should be safe with any lens that has the correct filter thread or to which it can be securely mounted and fully cover the entrance pupil.

One common misconception is that a longer lens concentrates more light; in fact, it's the other way around. A longer focal length makes a larger solar image on the sensor, therefore that image is dimmer (for lenses with similar entrance pupil diameter). The figure of merit here is the old f/ ratio. An f/8 lens at 100 mm will give the same image brightness as an f/8 lens of 20 mm, or one of 500 mm focal length.

Generally, the only concession you need to make for a longer lens is that, for a given focal ratio, the longer lens will have a larger filter mounting ring.

BTW, don't forget that your 2x converter costs you two stops of exposure (f/8 becomes f/16), the 3x costs three and a third (f/8 becomes f/24); combined, an f/8 lens is now f/48. With digital, your camera will meter through the lens, so you shouldn't need to manually compensate, but you need to be aware (in part, of why that combination isn't used much). You'll also give up some image quality with each converter, as well as lose a bit of contrast from the increased number of glass-air and glass-glass surfaces.