Opposition to the Empire not related to the Rebel Alliance?

Question

During the years prior to the Battle of Yavin (the early rebellion) there were a number of different groups that opposed the Empire. Many of these said groups would eventually join together to become the Rebel Alliance (and to a lesser, more ideologically extreme degree, the Partisans.) But while looking at the main pages of these rebel organizations, it came to my attention that surely there would have been other smaller opposition groups that either were not as successful as the Partisans and Rebel Alliance, or were crushed by the Empire.
So, were there any groups that opposed the Empire and that were not part of a major rebel organization, such as the Rebel Alliance or the Partisans?

I'll take legends or canon answers.

Answer 1

In Legends, yes. There were several noteworthy examples of rebels who were not Rebels.

The biggest outlier is almost certainly Garm bel Iblis, who was one of the original founders of the Rebel Alliance, but left after Bail Organa's death due to personality conflicts between himself and Mon Mothma. bel Iblis continued to wage war against the Empire with a private army of partisans, including a number of light capital warships (the famed Katana-class Dreadnoughts). Eventually, bel Iblis reconciled with Mon Mothma and was brought into the New Republic.

Several other factions are described on Coruscant in the years leading up to the Empire's fall. Some of them had ideological reasons not to join the greater Rebellion, such as speciesism. The Alien Combine was one such group: staunchly anti-Imperial, but also anti-human to a degree that would preclude joining the Rebellion. (At first; some of the leadership had a change of heart and aided the Rebel takeover of Coruscant.) On the flip side, Castin Donn's unnamed rebel group was both anti-Imperial and anti-alien. It's not known what became of the group after the Empire fell, whether they integrated with the new government, disbanded, or continued to agitate for a pro-human stance.