Skyglow filter may be a Bad Idea for general use, even if useful for faint emission nebulae, because of the light absorbtion - it's like cutting the aperture by approx. 30% and you don't have a lot to start off with. Personally I don't think you need any filters except for a green or neutral Moon filter (green would be better with the ETX80 despite the rather odd tint), plus a solar film filter for the Sun.