I wouldn't put the erectus in a 16'' high tank, there isn't enough hight for them to complete their mating dance and that can lead to issues like gas bubble disease in males. For seahorses a tank hight of 20'' could probably called the min, but 24'' is even better. One option you could explore is doing a dwarf seahorse tank, that tank would be plenty big for herd of them. Just be sure you do your research on any species of seahorse, the dwarfs require live food and you have to be able to provide that a couple of time a day.
Also seahorses dont like bright light, unless they have good hiding places. So you wouldnt need strong lights for your tank, which is nice because leathers (which are usually the favorite hitching posts) dont require tons of light.
It really doesn't matter how old your tank is, as how well cycled it is and set up for succuss.
For filtration, having good live rock and a live sand bed (remember you dont have to buy these, they are created by virtue of cycling your tank) will be the best filtration you can have. However skimmers are a really great thing to have because seahorses are "messy" fish. Some people do run canister filters with activated carbon to polish their water, which is great, others dont and is just fine as well. Having a good live bacteria community and doing regular water changes will be your best tools in maintain good water quality.
