Previously, I made the claim that "as far as The Silmarillion informs us, there are no apparent distinctions among Orcs of the First Age. There are just Orcs". This is both true and not true. It is true that, according to The Silmarillion, there is only ever one type of Orc suggested to exist. However, it is not true that there was only one such type of Orc in the First Age.
There is a very brief reference that suggests the existence of different breeds of Orcs in the Unfinished Tales:
But the Woodsmen were worsted, for these Orcs were of a fell breed, fierce and cunning...
Unfinished Tales: Narn i Hîn Húrin - 'The Coming of Glaurung'
As we can see, this passage suggests the existence of different breeds of Orcs. However, it is a very brief reference, and does not go into any detail about what the different breeds were like or whether they were named something particular, only that they were more fierce and cunning than typical Orcs.
The brevity of the reference, as is the case with much of Tolkien's world-building, leaves much to the imagination of the reader. This is both a blessing and a curse. It leaves questions such as 'how many different breeds were there?", "when did they first emerge?", and "did they look much different from typical Orcs?" unanswered. We could easily take these questions and expand upon them, creating a vast hierarchy of Orcish breeds, and saying that there was a precedent in the text. Yet, this would feel disingenuous, as though we were exploiting the text. After all, it is only a brief reference.
Therefore, we feel that less is more proper to take the quote at face value, that there is only a single other breed of Orc - the Fell Orcs. These Orcs, being more fierce and cunning than normal Orcs, will be a superior troop branch of Morgoth's legion, providing medium tier units. They will be the most common troops available to the Servants of Morgoth. That said, we will still keep the Boldogs. Their inclusion, even with the Fell Orcs, adds some much-needed variety and flavour to Morgoth's faction. Servants of Morgoth, then, will have Orcs, Fell Orcs, and Boldogs at their disposal, alongside other creatures of Morgoth's foul design.
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