I used a 35mm back on My ETRSi....it was the normal, not wide, format. And apart from being fiddley to load the film it all worked well and was even and flat. Of course wide angle lens became less wide due to the crop factor.
I would crop down my 120 film to 35mm panoramic if I needed the wider format saving the cost and transport of the extra film back and 35mm film.
Make sure your ground glass screen has the 35mm markings on it, mine did and it made shooting 35mm so much easier.
The Etrsi is a lovely, portable, underated medium format system, enjoy.