I'm interested in this myself, so checked specs and measurements.
The ES erect 8x50 finderscope has a tube diameter of 54.9mm, according to the Agena Astro product page:
http://agenaastro.com/explore-scient...ed-finder.html
My SkyWatcher 9x50 finderscope (that comes with the standard bracket) has a tube diameter of 53.1mm according to my digital caliper.
Looking at the rubber ring and minimum internal diameter of the bracket, I have doubts that the ES finder would fit in the SkyWatcher bracket - I think it might be too tight a fit. But, maybe worth the chance anyway. I think I'll roll some dice, or toss a coin, to make up my mind.
Edit:
After deciding a coin toss was too risky, I pulled apart the SkyWatcher finder and measured the bracket internal diameter - there's a recess that the rubber ring sits in and behind that is the narrowest part of the bracket - the largest diameter of the narrowest part I could measure was 53.85mm, so it looks like it won't fit.