Smaller ones do exist. I have an 8 or 10gpd I forget. It came from sears. Not that Sears is the best place to get an RO for a fish tank, but I picked it up on clearance for pretty cheap.
I also get consistant TDI of about 2, without a DI, so I'm fairly happy with it.
Just very slow to fill a 5 gallon bucket for water changes and such. So I think I will be bumping that up to a higher flow when its due for replacement.