I agree with the above but it should be cycled after 6 weeks. Still, check your water parameters - you should have 0 ammonia and nitrItes and nitrAtes under 20. If you have more than 0 readings for ammonia or nitrIte, your tank needs some more time to cycle. If your readings are ine but you have high nitrAtes, you need to do some more partial water changes (always with de-chlorinated water). Read through the links in my signature if you don't understand any of this stuff or why you need to do it.
Anyway, the zebra danios would rather keep the upper ayers to themselves - I would avoid adding most gouramies to this tank (though it could have been a great choice). The mollies, depending on exact variety, will get pretty big themselves - probably about 4" (approx. 10cm). If you have a male and female, get another female before you add anything else (but only once you're shure the tank is cycled).
Then you have room left for something else...
Seeing as this is your first tank, stick to hardy species that are easy to take care of. Perhaps something to fill out the bottom layers of your tank would be welcomed - cories, most any species, make an excellent choice (but the biggest only get to about 3" - bronze, albino and peppered being the larger hardy ones). If you want something that'll look a little more impressive, perhaps look into getting some zebra loahces. However, you must then make sure you don't add anything that's likely to get nipped as the zebras can sometimes be nippy. With both the cories and zebras (I would personaly suggest you avoid mixing the two BTW), it's a good idea to get a group of at least 4. Kuhlie loaches are another one to consider but you won't see them as often - they get to 4" (approx. 10cm). The loaches are relatively hardy - but the cories I listed are hardier.
Rainbowfish - but only the smaller species - when kept as a school are very impressive fish. You could get a group of 6 or so dwarf neon rainbows (melanotaenia praecox). They do not get to all that much over 2.5" at their largest but they become very brightly colored and active once settled. There are larger species but they are more suited to a longer tank due to their activity levels and their need for a school.
Swordtails are another choice - they are also livebearers, like your mollies, with males growing to only about 3" but females get to 4", eccasionaly even more. If you go for these, stick to a single male with 2 or more females. There are many lovely varieties to choose from (though the wild 'green' form is my favourite - and also the hardiest).
Assuming you don't add anything that's likely to get bullied and no bottom-dwellers, you could also try some kind of dwarf cichlid pair. For these kind of fish, a little extra research would be called for (though you should research ALL your fish carefuly BEFORE buying them). The most commonly kept dwarf cichlid has to be the german blue/gold ram. These are relatively peaceful and could work but they can also be fragile. If you decide on this fish, add it after at least 6 months when your tank is mature. They don't get much over 2.5" but are very brightly colored and have lots of character. Alternatively, the bolivian ram is a more hardy choice - but you should add them last as they can be aggressive. With the german blue/gold rams, it's best to keep a pair, with the bolivians, it may be better to stick to a single one to avoid aggression issues (either with each other or if they decide to breed). As I hintd at at the beginning of this paragraph, it's also generaly best to not keep other bottom dwellers if you want to avoid aggression. Things like kuhlie loaches which stay hidden most of the time should be ok though. Cories would be ok with a single bolivian ram but only if they are in a group of 6 or more (the cories, not the ram!). Zebra loaches can usualy hold their own so would be an ok choice (though you would likely see a little aggressive interaction and this could be stressful for the more fragile german blue/gold rams).
