OK, for a 20 gallon, with that stocking, you could go with:
1) 5 or 6 Botia sidthimunki - This will set you back over $100
2) 3 Botia striata
- I have 8. They're every bit as engaging as clowns, but they stay small - 4 to 4.5". You never will have a snail problem either. They're relatively inexpensive.
I would not recommend any other loaches in this tank. All of the peaceful loaches like to be in a group. 3 minium, 5 or more is better. Kuhlis are wonderful, but you'll never see them unless you have six or more, they tend to die for no apparent reason (they're like ottos that way), and they really like sand. Botia kubotai
tops out at 5", B. almorhae at 6". You really don't have room for 3 of either of these. B. morletti is a king-hell fin nipper - best in a species tank, next best in a tank with other semi-aggressive loaches.
And, fo the record, I am aware that the Genus Botia has been changed/split out. I know tht icthyologists need job security too, but they should find it with deep water lanternfish and leave my loaches alone. :P
That's my $0.02. Try not to spend it all in one place.