If it is sucking on another fish this seems to suggest that the loach has parasites or some other skin disease that the syno is keeping at bay....
You never know with synos anyway, sometimes they are the most peacful fish in the world, whilst other times they are extremely terratorial and eat small fish.
I have 3 synodontis petricola which are of the dwarf species of synodontis and only grow to around 3.5". These are a perfect species for the smaller tank but aren't the most hardy of fish.
I also have a 7" Synodontis Angelicus who rules the roost in my 57G tank. Surprisingly he is incredibly tame and does not bother any of the other fish. I have only seen him go for one fish which was my snowball plec which had very similar markings to the Angelicus, so the plec was moved into the 50G Loach tank.
Anyway, Euruptus are usuallygreat species but be careful as they do get bigger! you should have at least 30G and keep them in small groups (3 if possible).
Ben