No, I agree, no problem with keeping a pair of females together. (unless a female changes to a male then you have a legit pair)
Your biggest problem is that a 20 tall is NOT a large enough tank for two seahorses. Minimum size would be 29g for just one pair.
Smaller tanks are very difficult to maintain water quality due to the eating habits of the seahorses which even with the larger tank need much better husbandry and more and larger water changes than you would need for a reef tank. Insufficient water quality means bacterial infections down the road. Test kits available to use cannot tell us when the water is going to promote the bacteria to plague proportions so we need preventive maintenance.
Here in Canada we no longer get wild caught, and I think in the US it is getting like that as well in most areas. My experience was the wild caught were the cheapest sold with imported tank raised being a bit more expensive, but true captive bred are definitely the most expensive.
Most common seahorses now are basically net pen raised or raised in large cement tanks where untreated ocean water is the source of parasites they get introduced to and may cause problems down the road.
Best to buy is true captive bred and directly from the breeder is best.
If a store buys captive bred and then places them in their store system hooked to other fish in other tanks, then the CB will now be exposed to parasites from the other fish which too might cause problems down the road.
These "tank raised imports" are trained onto frozen mysis just like the true captive bred, and it's not unknown for a store to claim the tank raised to be true captive bred so be aware. Many times the store doesn't even know the difference.
