They are quite a challenging fish to keep and although you may have read a 20 gal
is enough, I would suggest a much larger tank. Pipefish get quite large and although they do not swim around much it will still get quite cramped. They do indeed do better in a species only tank or at the very least extremely docile tankmates.
If you have had good experience keeping seahorses, you might have some decent success keeping these fish. If not, I would suggest you pass. They require alot of daily care, live feedings and high water quality.