Long before a single copy of the NWT was printed Jehovah’s Witnesses taught that Jesus is the unique one of a kind direct creation of Jehovah The Almighty God.
The verses in Colossians 1:15-17 do not contradict that belief. Translating the word for “all” is dependent on context and there are a lot of instances of it being modified to demonstrate that the original writer did not intend “all” meant absolutely all.
For example Luke 13:2 in the Greek original does not modify the word “all” yet it is modified by the following translations to name a few
"all the other Galileans" - NIV
"all other Galileans" - NASB
"all other Galileans" - NAB ('91)
"all other Galileans" - NRSV
"all other Galileans" - NKJV
"all the other Galileans" - RSV
"anyone else in Galilee" - NEB and REB
"than any other Galileans" - JB
"than all other Galileans" - NJB
"any other Galileans" - AT
"everyone else in Galilee" - CEV.
"all other Galileans" - TEV.
"all other Galilaeans" - BBE
"other people from Galilee" - GodsWord
"all the other Galileans" - ISV NT
"the rest of the Galileans" - Moffatt
Commenting on the reason that NWT adds the word “other” this article in Watchtower said
In considering this matter it is well to note that Bible writers often took for granted that certain things would be understood, just as writers in our day do. For example, the apostle Paul states, as we read at Colossians 1:16 that by means of Jesus Christ all things were created in the heavens and on earth. But since we know from Revelation 3:14 that Jesus himself was also created, the New World Translation adds the word “other,” which clearly is what the apostle had in mind.
But even here, it might be added, that, were it not for the prevalence of the trinitarian teaching that Jesus was not created, it would not have been necessary to add the word “other.”
Thus also the apostle Peter at Pentecost quoted the prophecy of Joel 2:28 (RS): “I will pour out my spirit on all flesh,” and applied it to what took place there.
However, we note that God’s spirit was not poured out literally upon all flesh at that time. But it was poured out on ‘all sorts of flesh,’ that is, on not just a select few as in times past but on sons and daughters and men- and women-servants.
And so the New World Translation reads that the spirit was poured out “on every sort of flesh,” which obviously is what is meant.
Acts 2:17, 18
This blog has an extensive discussion on the topic including this example
The Bible writers very often excluded the subject of a clause - or the person speaking/writing the clause when using the term “all” (and “every”). This is a common usage even today. For example, the police sergeant making an arrest of a criminal group might say: “Everyone in this room is under arrest!” - (person speaking excluded.) Obviously the sergeant does not include himself (nor his captain who is with him) even though he says “everyone”! Or “the criminal tied up everyone in the room before stealing the gems.” - (subject of clause excluded.) Obviously the criminal didn’t also tie himself up.