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Hi guys,
I was wondering if I could put a pair of GBR's, 2 cory catfish, 1 pearl gourami, 1 viper shrimp, and 4 cardinal tetras in a 20 gallon tall. Also would a trio of Cockatoo cichlids (1 male/2females) work instead of the GRB's.

I was wondering if I could put a pair of German Blue Rams, 2 cory catfish, 1 pearl gourami, 1 viper shrimp, and 4 cardinal tetras in a 20 gallon tall.
yes
along with 20 more fish too

1 viper shrimp ... you mean wood shrimp: Atya or Atyopsis
http://nanocaridina.weebly.com/uploads/ ... 8_orig.jpg

Also would a trio of Cockatoo cichlids (1 male/2females) work instead of the GRB's.
yes
you could have those INSTEAD
but
NOT in addition to

the cichlids are territorial in my experience
if you had a bigger tank you could have both types together

Should I add more cardinals/Cory cats, or another species? Also I forgot to mention that I have 1 Kahului loach. Would this be a problem with the cockatoo cichlids.

Should I add more cardinals/Cory cats, or another species?

anything you want

Also I forgot to mention that I have 1 Kahului loach. Would this be a problem with the cockatoo cichlids.

not at all