Firstly, manufacturer's recommendations are so poor, that it is worth disregarding them when it comes to stocking.
Secondly, I would recommend you against putting more fish in the fish tank for the following reasons:
* Danios and cardinals are already unsuitable for a tank that small because of their adult size and activity level, adding more fish will only cramp them more
* Danios and cardinals are both schooling fish so should be kept in groups of 6+ (which is already not possible due to above reason.
Ideally, for a tank that size, I would recommend (for example, one Betta and 8 espei rasboras (or 8 ember tetras instead of espei rasboras) because those schooling fish only grow to 2cm (which is less than half the size of the ones you have now).