I have a False Percula Clown in a 20L who currently lives with a Yashia Hase Shrimp goby and a Yellowtail Damsel. Clowns are a good beginner fish, but I'll also recommend these in a 20g:
-Atlantic Pygmy/Cherub Angelfish
-Coral Beauty Angelfish
-Flame Angelfish
-Halfblack Angelfish
-Rusty Angelfish
-Convict Blenny (note: these guys do not stay 4 inches like book says. can get up to 12" easy.)
-Midas Blenny
-Redspotted Blenny
-Redlip Blenny
-Bangaii Cardinalfish
-Orangestriped Cardinalfish
-Flame Cardinalfish
-Pajama Cardinalfish
-Clarki Clownfish
-Maroon Clownfish
-Orange Skunk Clownfish
-Pink Skunk Clownfish
-Tomato Clownfish
-Dwarf Golden Moray Eel
-Bar Goby
-Blue Gudgeon Goby
-Green Mandarin Goby [with caution, not for beginners]
-Yellow Target Mandarin [with caution, not for beginners]
-Blackcap Basslet
-Arc Eye Hawkfish
-Longnose Hawkfish
-Bluespot Jawfish
-Dusky Jawfish
-Fu Manchu Lionfish
-Fuzzy Dwarf Lionfish
-Dragon Pipefish [with caution, not for beginners]
-Banded Pipefish [with caution, not for beginners]
-Bluestripe Pipefish [with caution, not for beginners]
-Bennett’s Toby
-Valentini (Saddleback) Toby
-White Spot Toby
-Carpenter Flasher Wrasse
-Filamented Flasher Wrasse
-Fourline Wrasse
-Longfin Fairy Wrasse
-Redfin Fairy Wrasse
-Tricolor (Lubbock’s) Fairy Wrasse
-Twinspot Hogfish
Where your just starting out, Id go with 2-3 fish, then if you want when you feel confident, add one more small fish if you want.
-LJ