Hi,
Clown loaches will be fine with guppies, my loaches are in with tiny fish no problems (they eat platy fry though so would probably eat guppy fry). A lot of places recommend larger fish to live with clown loaches as they grow up to 14 inches long.
As clowns can live for 40 years and most small tetras live 5-8 years i plan to stop replacing small fish as they die off then eventually have a group of 6 or so rainbow fish with just the loaches and which ever fish are still around. My biggest clown is only 5 and a half inches at the minute so it will be a while before this happens.
Emma