IS it possible to find the caped bullets at all, and unlock "The Bullet" with only the Robot's past completed. And no others finished yet? Thanks

This is the situation I am in at the moment and know about the process to get him, however I have been playing quite a bit since I beat the Robots past and have yet to see a caped bullet kin. I may have accidently killed any that might have spawned, and since they are so rare it would be hard for me to figure out without luck when they die so easy. Hence why I am asking if anyone knows for sure so I can stop wasting my time if they do not spawn in this situation lol.

Almost forgot to mention, the run I did when completing the Robots past was a rainbow run also, incase that makes a difference.

Thanks ahead of time for any help.