6g's is just to small. If it where a little bigger and you could do a chiller to drop the temp to like 65F you could try caps, but even caps at 6g's is not enough room.
The juvie idea is noble, but problamatic in the continual changing of syngnathid species. Do a search on mixing species, you will be combating the same elements even if they are all the same species, just by constantly changing horses.
Even the 12's are to small they really hold closer to 8g's. If you had a 24 you could try for tubers or fuscus if you could find them. Unfortanetly those cubes just don't make the best seahorse homes.
I'd love one if they did believe me.