Tetras and guppies come from different kind of waters, most tetras want soft an acidic water, while the guppies want it hard and alkaline. Not to mention the differences in temp The guppies want between 21-25 degrees, and tetras want it warmer, around 24-26 degrees

This depends on whether you let your tank cycle completely before introducing the fish. If all the ammonia and nitrite is gone, it should be safe to introduce most of them, but you should still be alert the first two weeks or so, and test the water often and do water changes when necessary Otherwise, I guess you know the whole bag-process d:

What I was thinking now was that i would get guppies and mollies (i've read they go well together)

How many of each should i get if they were to live comfortably in a 20 gallon tank?

Mollies do thrive in brackish water, but they live perfectly well in freshwater tanks, and they have similar needs as guppies
Either way, you could get a bunch of male guppies if you don't want them to reproduce, because they do, in a crazy speed! xD Other than that, you can easily calculate the stocking level by giving about 2 liters of water to each 1cm of adult fish body. Which is about 1 gallon per inch It's different with guppies and other small/shoaling fish, where you could even think 8 inches per gallon. Guppies grow to be about 2 inches in length, and mollies to be around 3-4 inches.

In a twenty gallon, you can typically fit 20 inches of mollies or 160 inches of guppies. So you could fit 5 mollies and around 20 guppies if you like It could be good to have some bottom dwellers in such a tank as well! Like a few corydoras or common ancistrus would stay pretty small Then it would be good to have maybe... 2-4 corys and 1-2 ancistrus. The ancistrus will be 4 inches, and the corys will be around 2-3 inches when grown. So if you would want the bottom feeders, you could fit 2 corys and 1 ancistrus + 3 mollies + 12 guppies. How does that sound?

But, you shouldn't get all the fish at once, remember that! You could start with getting all the guppies for a start so the filter will get used to fish After a week or two with good water quality, you could buy some mollies

Since a molly grows to be 3-4 inches, it will need 3-4 gallons. Well, I mentioned IN THE PREVIOUS POST, around 5 mollies and 20 guppies. But if you like, you could have a few less guppies to make room for potential fry.

No, you count guppies as 8 inches per gallon, like I also mentioned. But if you feel uneasy about the mollies, get yourself 3 for a start or something d: