However, there are other things to consider as well. Personaly I'd say go for what you have planned now. Cories BTW, are often counted as half their size which makes them 1.5 inches and pearl gouramies breath air. Male guppies only grow to about an inch, females to 2. Platies generaly grow to a max. on average of 1.5 inches. So what you get over-all is approx. right for your tank. The fact that it is a hex would normaly decrease the number of fish because there is less oxygen in the water. However, with good filtration and an airstone and keeping in mind that cories do take air from the surface when necessary and gouramies NEED to take air from the surface (they drown otherwise), your tank should be ok. The very best way to see whether your tank is correctly stocked is to measure your nitrAtes every week before a water change (once your tank is cycled). Provided your tap water doesn't contain high nitrAtes you should be able to keep the nitrAtes below 40 with a single partial water change per week. If not, you either need to keep your stock numbers lower or increase water changes (this isn't to say you should put youself in a situation where you need a water change per day though - the fish wouldn't have enough oxygen or swimming space if this were the case - the nitrAtes only indicate your tanks ability to keep your water in good condition with the specific number of fish). Watching your fish and making sure they look happy and healthy also helps.
Oh, BTW, I think a pair of pearls are a great choice! They are my absolute favourite fish!