First of all, with a bio cube, you may have trouble keeping the water temperature in the recommended range of 68° to 74°F.
While seahorses may live in their natural habitat in much warmer waters, the water is always changing. In our tanks, bacteria multiply exponentially with each rising degree above 74° as the water is not changing.
Substrate on the bottom is a personal choice with many using some form or another and others like myself preferring to go bare bottom.
General rule of keeping corals is to not keep any stinging corals and to be sure to keep corals that can handle the cooler temperature range of the seahorse tank.
Most people keep their seahorses in water with s.g. between 1.020 and 1.026 but they can live in lower s.g..
seahorse.org has an article listing hazard levels of various fish but for your tank you may be listed to certain blennie type/size fish.
A 29g tank is the MINIMUM recommended size for up to a pair of seahorses of normal size.
I hope you mean by "this weekend" that you are just going to start the biological filter, and not intending to put seahorses in already. I'm assuming the tank doesn't have an established biological filter when purchased, or if it had, the probability that there was die off when moving the system.
When purchasing your seahorses, keep in mind that your best chance of success comes from purchasing true captive bred seahorses, either from another hobbyist raising them, or places like seahorsesource or seahorsecorral.
Also, the more different species in the tank be it other seahorses or other fish, the less your chances of success may be due to the propensity of seahorses to succumb to pathogens that they have not grown up with but are introduced to them by other tank mates.
