Hi,
In a tropical aquarium the generally accepted maximum stocking level is about 2" of fish (excluding tail) per gallon. Therefore in a 30 gallon tank you can have about 60" of fish - added over a period of time not all at once. I have gone a lot over this limit in some tanks, but it is best to start with good intentions
Guppies - absolute max size 2"
Mollies - absolute max size 4"
Danios - from about 2" (zebra) to 6" (giant)
So you could haver a good number of guppies with your mollie and a shoal of danios.
You can add a lot of different types of community fish to this mix, such as corys (bottom dwellers), hatchet fish (surface dwellers), tetras (middle dwellers), small plecos or otos (for algae), rainbowfish, some types of gouramis, etc.
Probably best to see what you like and then ask whether they will mix in. People would need to know how many existing fish are going in the tank to be able to work out whether you have enough space.
As for the Penguin, we don't get those over here in the UK, but looking on Drs Foster & Smith it seems to be a reasonable looking H.O.B. filter. hopefully some with some experience of these will post a response.
HTH, Eddie