Dream Blue Velvet shrimp are really just another colour morph of Red Cherry Shrimp. I find that cherry shrimp in general only get around 3cm but that can take some time usually I have seen them smaller and suspect that over crowding in nano tanks or simply excessive hormones in the water from so many shrimp keeps their size small. It could even be genetics, perhaps size is linked to set colour genes.
I currently have a real monster of a wild type cherry shrimp she literally dwarves all the other cherry shrimp in the tank and would easily be over 4cm in size. I have no idea why she is so big, it could be genetics ("hybrid" vigour so to speak since she is a wild type throw back but technically is not a true hybrid since she is not resulted from a cross between two species), it could be her age, since shrimp keep growing for as long as they live with every moult seeing them gain a little more size. It could be that she was born and bred in a pond that had optimal shrimp needs, with more natural foods that might be other wise provided in a tank.
Personally I have discovered that the different fancy colour morphs of cherry shrimp don't always do well once removed from the original breeders tanks and placed into other peoples tanks. You would not think it but the chemical change in the water can have a HUGE impact on the shrimps colour, and it can happen surprisingly quickly. I have tried a couple of times to keep yellow cherry shrimp, with failed results they just never took to my water. Rili shrimp only did well when I bred my own line from a couple a fluke rili's that turned up in my all red tank. I would love to try blue but they are even trickier to keep and maintain.
I would not necessarily blame the fish for the failure of the Blue velvets to thrive in your tank, it could be that your water is just too different to the original breeders and the shrimp all though not dying immediately from the different water or showing any discomfort just fail to thrive and breed. If you do try Blue velvets (or any other colour morph) I personally would try to get some from a local breeder, especially if they can inform you of all their water chemistry and you can then try to best replicate it perhaps in a dedicated shrimp tank rather than messing around with pH, gH etc on a grand scale in a tank that is already happily housing other shrimp and fish.
I am on a shrimp only forum which is also "local" as in the members are mostly from Australia but these members are incredibly helpful and eager to help budding shrimp keepers, they often will even help out with what they consider culls giving them away to help people with less money break into the other colour strains without breaking the bank. There are a number of ways to locate local shrimp breeders, one is via social media places like Facebook, Gumtree, or Craig list or even just search for some shrimp forums that are based in the United States. Personally I would join a dedicated forum since that way you will hopefully avoid some of the dodgy dealers that can proliferate on certain sale sites. Forums at least have rules, MODs and most members are quick to report any negative dealings they have had with other members.
