Hi, with adequate filtration, heater etc a few guppies and shrimp sounds ideal for a tank that size to me.
From experience i havn't found any shrimp brilliant at 'cleaning', your best bet is a snail for keeping the glass clean...cherry shrimp do graze a little on some algae...but from my experiences, not a patch on a snail.
But on the other hand shrimp are a lot more active!