For the most part, my story is told from the main character's perspective. However, there are a few scenes that I would like to write that really don't require the main character to be there. I could maybe come up with some ways to place the main character in the scene, but I can only think of some ham-fisted ways.
Is it okay to switch perspectives, or would it feel too abrupt?
It depends to what degree you change perspective, if you're switching from one first person narrative to another you can do this smoothly enough. Switching from first person to third can be, and often is, disruptive or jarring to the narrative.
The only other thing I would note is that writing the whole story from a single first person perspective except for a couple of random scenes, that can be really hard going on the audience.
I don't think 'abrupt' is the right word to describe the potential issue.
Changing perspectives will be jarring if it isn't handled properly, but as long as you make it clear within the first few sentences that you have changed perspective, then I don't see that to be a problem. There are many, many books that are written from multiple perspectives.
I think what you're possibly more concerned about is that having the vast majority of your story told from one POV and then just a few random chapters or scenes from someone else might make it feel imbalanced, and I do think this is more of a possible issue.
I once read a published thriller novel by a famous author that had a single scene from an alternate POV. I did find it very jarring, but it was exacerbated by the fact the POV character was deliberately obfuscated in order to try to mislead the reader. The whole thing was badly handled, in my opinion, but I digress.
My recommendation would be to try to find ways to make your story more balanced, probably by expanding on the scenes done from the alternate POV. Could you make it so there are at least three scenes from one alternative POV? Or possibly three each from a different POV. Three is the magic number for making things feel complete...
As an aside, I definitely don't think crow-barring your main protagonist into those scenes sounds like a good idea.
Anyway, all of this is very subjective, but there's my opinion.
As is often the case, it depends on your book, writing style and even genre.
If your book is continuously a single PoV and then you change perspective, it might be jarring for the readers, because they expect the text to be from the previous PoV and then might get confused. Readers have expectations about what they are currently reading and it can be difficult if those expectations are suddenly broken.
If switching PoVs happens more frequently, you only need to have the proper introduction for the paragraph and the reader will understand that it's another PoV change. Switching PoVs frequently is common and often expected in some genres, so it's not bad as such. It might just be bad if the new PoV character isn't indicated in the first sentence in some way.
In the former case, where your readers don't expect it, you might have to emphasize that you switched PoVs, more than by just starting the paragraph with "Person B followed the moskva down to gorky park." Such an introduction could get understood as the protagonist observing Person B following the moskva, not as a PoV change. Using a header for the paragraph might help, though might not be the best option if your text doesn't use headers anywhere else.
Some authors use a separate PoV for the first scene in each chapter and the rest of the chapter is back to the old PoV, which is also something you can use to help the reader find the pattern required to understand your PoV changes.
In any case, you need to make your reader understand from the first sentence that he's no longer watching over the shoulder of your main protagonist.
What you should not do is write characters into scenes just so you can use your existing PoV. If the character has no reason to be somewhere, he shouldn't be there and finding contrived reasons for the protagonist to be there erodes the suspension of disbelief. You can, however, have someone who was there talk about it with your protagonist and so provide the same information indirectly.
An omniscient narrator can tell the whole story: sometimes the narrator's focus is on the MC, sometimes it's elsewhere.
However, if the narrator only rarely strays away from sitting on the MC's shoulder, as it where, the effect could be a bit confusing. When it's done a lot throughout the course of the novel, readers expect it. If you're only going to do this once or twice, you might want to consider the alternatives.
An alternative solution (not necessarily a better one, but one you can consider before discarding) is having another character tell your MC about the scene post factum, rather than the narrator telling it directly to the reader. Roger Zelazny makes use of this in The Amber Chronicles (narrated in first person throughout), and Tolkien has part of the Paths of the Dead episode, where none of the hobbits are present, recounted to the hobbits later. (This also works best with the order things are told, in this particular case - an event can be a surprise, to the readers and to the hobbits, whose POV the story mostly follows. Then comes the explanation of how it happened.)
