A good setup for a 20 gal tank?

I would suggest less male guppies an more females or else the males will stress out the females. A nice ratio is usually 1-3. The betta may or may not go after the guppies. I've had some that did and didn't. I should say however that many people see it as a problem. I'm not sure what a pencil fish is, but everything else will be ok.

some of the fish like the swordtails are considered Tail and fin nippers and may chase your betta around, Keep an eye on it and have a backup solution handy just incase

I agree always have less males then females when you have livebearers. The betta may or may not work out, it depends on the individual. I also do not know anything about pencil fish but I would reasearch them farther before getting one. I have a feeling it would outgrow a twenty but like I said I really don't know. How bout this?
4 female gups 2males, two female swords 1male, male german,5 corys(a dwarf species would be best), and the betta, you could add lots of stuff your preferance such as a school of tetras etc. But remember it is better to understock then over.