Okay, that sounds fine.
A 28-gallon biocube can make a fine seahorse habitat if it is properly equipped and aquascaped, Sasha. A 3 in one filtration system can provide efficient biological filtration and compact florescent lighting is generally a good choice for a seahorse setup.
I think the primary biofilter will provide the aquarium with sufficient circulation for your needs without having to employ the powerheads. If you use the powerheads, only one of them will be needed at most, since, with an output of 265 gallons per hour, one of them will turn over the entire volume of the 28-gallon aquarium about 7.5 times per hour, and seahorses do best with a turnover rate of 5-7 times per hour, in my experience.
The 3-in-one filtration system will provide pretty fair circulation all by itself, so just one of the powerheads is all you should consider, and you may be just fine without any powerheads at all.
To get you started off on the right foot, Sasha, I will send you a complete copy of the Ocean Rider Seahorse Training Manual as an attachment to an e-mail so that you can download it, save it on your computer, and carefully read through the material at your convenience. The seahorse training manual is very comprehensive, consisting of several hundred pages of text with more than 250 full-color illustrations, and it will explain everything you need to know in order to keep Ocean Rider seahorses successfully in a home aquarium.
Best wishes with all your fishes!
Pete Giwojna, Ocean Rider Tech Support