Have fun! I Stage Managed The Clean House two years ago and it was a great experience - the visual elements in the show were stunning!
The only difficulty in calling the show that I remember is the subtitles that translate the Portuguese because it happened quickly and I wanted to make sure that we stayed on track with the translation (and that the actors who didn't speak the language also stayed on track). Speaking of language, having a dialect coach for the Latina women was very helpful (unless you have actors that already know the language and dialect)
Make sure you have items that can be cleaned up and reset in time to be used in the "operatic mess" - we used various potpouri, the chess set, ribbon, and several larger items that could be thrown or knocked down without breaking anything. Also - Mr. Clean magic erasers were invaluable for the inevitable smudges on the white walls. Oh, and get a back-up fish (you know, just in case...). We alternated ours depending on who was acting more "frisky" that day.
As far as the quick change at the end, we did not try to hide it. The actors stayed on stage and in position while Matilde speaks - Charles took off his coat and Ana took off her robe (I also think that Charles had a flower in his vest that he gave to Ana to put behind her ear). It was kept very simple and they had their earlier parent costumes under-dressed.
Hope that helps some; have fun with the show!
