It depends on the when and the where.

The scenario you describe is actually pretty unlikely. Not because people wouldn't have been shushing, but because there weren't dedicated movie theaters in 1900. Instead, there was a wide range of exhibition practices, including private "peepshow" viewings with the Mutascope and related techs, temporary shows as tent "attractions" at carnivals and fairs, and what today we'd called mixed-media practices, as with films shown as one part of a vaudeville variety program, or, in the case of early travelogues and passion plays, accompanied by a live lecturer in a church, fraternal society, or possibly school.

Details are scant, but with the possible exception of the latter scenario – where an actual person is talking – it's hard to believe anyone in the audience would have been shushed.

Of course, as in RDR, there would have almost always been live musical accompaniment, especially once the first permanent movie theaters – the Nickelodeons – began showing up around 1905. But even then, this was largely a working class entertainment, as much about socializing as about spectating. It's likely that there was a fair amount of ambient conversation and even some catcalling and the like. What we can say for sure is that films in this period didn't have fixed starting and stopping times; they instead played on a loop, and it was expected that patrons would arrive at arbitrary times, come in "in the middle," and then stay (if they felt like it) until they reached the point where they came in. And maybe then stay for another loop, why not. So certainly there would have been a great deal of bustle, of constant comings and goings, and it's hard to imagine a baseline of attentive silence that anyone would have felt broken.

That really only showed up with the rise of the Movie Palaces in the mid-1910s. These massive, glitzy theaters capitalized on (and also created) the growing middle class interest in the movies – and they did so precisely by overlaying the filmgoing experience with the conventions of the bourgeois (live) theater. Now you had to buy a ticket, you had to get to your seat by the starting time, and, crucially, you were expected to uphold bourgeois public decorum: namely, quiet, focused spectatorship. To put it another way, this was the point at which the movie itself became the central (sole) focus of the moviegoing experience. Silent attention was the expected "respectful" attitude to take.

Now, would people have actually been shushed in the movie palaces? I can't really say. I would hypothesize that yes, in the big city theaters with full orchestras, they well might have been – just as you might be shushed at a classical music concert. In smaller venues with more local populations, it seems less likely to me. Hearing obviously wouldn't be essential to the meaning of a film until the synchronized sound era, post-1927, and even then, silent viewership isn't the norm even today at all times in all places. (I still fondly remember some of the things drunk ladies were yelling at the screen during Magic Mike, for instance.)

On the other hand, big hats were apparently a big problem. (Parenthetically – I'm glad I dug this up! Made in 1909, it shows the kinds of lively theatrical behavior – albeit in a comedic mode – that contemporary audiences would have presumably taken for granted.)

Source: Charles Musser, The Emergence of Cinema; Eileen Bowser, The Transformation of Cinema; Rick Altman, Silent Film Sound