Not sure about bolivian rams... they can be quite territorial so you may have aggression problems due to the tank being small. Perhaps a pair of one of the smaller apistos instead or a single keyhole depending on dimensions.
As for Plecos you could have Zebra, Clown, Goby or possibly bristlenose
Your title has me singing pirate songs..."Ohhhh...what shall we do with the 18 gallon, what shall we do with the 18 gallon..."
Anyway...as a total puffer lover I suggest a dwarf puffer as the main attraction (maybe two if you can be sure to get a male/female pair) and they get on with bottom dwellers just fine.