@ Fishie, BN Plecos get to be around 6" and are too big for a 20G, maybe a ABN which reaches 5", but still too high of a bioload to let caitlyn get the other fish
tiger barbs need to be in groups of at least 6 say most sites, but in most cases it should be at least 8 which is too many for a 20G as they get to 3" and need a lot of swimming space......
A red tail shark is too large as it reaches more than 7" and needs a minimum 55G
If you are looking for a colorful tank, and for it to be peaceful with no compatibility issues, I would do something like this, just an option.........
1 dwarf gourami(any type of dwarf species)
3(a trio) of small corys(such as pandas, skunks, etc.)
A few platys(your choice)
6 Schoolers(harlequin rasboras, neon tetras, both are colorful)
Another option is to make a barb community tank, BUT NO tiger barbs, other species instead.....so this option......
6 cherry barbs
6 gold barbs
A trio of corys(small)
And you are done, you can keep temps. lower too, and save money on heating as they can live in 74F and don't need temps. high as 86 as some fish do.
hope that helped