The Andorian Incident can be safely watched, but only because it is a beginning of an arc. The Shadows of P'Jem and Cease Fire are sequels to The Andorian Incident.
I think "standalone" refers to a single episode that can be comprehended as a self-contained story -- i.e., beginning, middle and end, protagonist and opponent, our hero/heroine working toward a goal but running up against obstacles, final climactic confrontation -- without the need to know previously established information that is
critical to understanding the plot.
One of the ongoing challenges of a series is to be able to subtly "re-up" the theme and overall goals of the show, as well as provide context for a "follow-up" episode, in order to ground a viewer who may have missed earlier shows. If that context can be established in an early scene or two, with a few key lines of dialogue, then the viewer will likely be able to follow along with the episode. Sure, one will miss nuances and passing references to previous episodes, but hey, that's a reason to watch the entire series.
The Expanse, The Anomaly, Impulse, Proving Ground, Stratagem, Similitude and Twilight are all parts of the Xindi arc. Some of them (like Similitude or Twilight) focus on a self-contained story, but they're impossible to understand without the context of the Xindi arc.
Impossible? Not necessarily.
Since "The Expanse" is the episode that sets up the Xindi arc, it can be viewed as a standalone. Key scenes such as the Archer/Trip drinking scene and the T'Pol/Soval scene re-up the ship, the crew, the relationships between the characters.
For the rest of Season 3, one can usually pick up the basics -- we're in an unknown region of space, it's dangerous, we're at war and Earth is depending on our crew -- in the opening minutes, as the episode's plot gets underway. "Similitude" is about Trip being gravely injured and extreme measures being taken to save him. "Impulse" is about being trapped on a derelict ship with a crew that has gone mad. "North Star" is about unexpectedly finding descendents of long-lost humans on a faraway world. "Extinction" is about some of our crew being infected with a unique virus, and becoming targets of another race determined to stamp that virus out at all costs. "Doctors Orders" is about Phlox being left in charge and virtually alone on the ship.
Interesting that many folks consider the two-part "In a Mirror Darkly" to be one episode, but the three-part Vulcan arc to be three episodes...
