I don't think I'd put any pipefish in a 20 gallon tank with a sixline.
The reason everyone loves bluestripes is that even if they don't eat frozen, they can usually do ok eating pods in a small tank. Bigger pipes have a harder time and are (IME and according to my research) less likely to eat frozen foods.
Even assuming your sixline isn't too aggressive (more likely in a small tank, esp. as it gets older), it's going to compete for pods.