BAS in Bolton have some lovely eels in at the moment!
I have a Zebra Spiney Eel and a Half Banded and they are fantastic fish.
Well the eel I got from them was a zebra spiny eel, and none of the eels you mentioned (if they have been IDed correctly) will get anywhere near 18". They will stay under 8" IME.
The two yellow tails or barred as they may be known (M.pancalus) that I had for arround 9months or so grew to arround 6" or so, I would say that 8" would be the max they would get to IME of their growing behaviors. It will vary though depending on what you can get them to eat etc.
Just make sure you ID it properly, but its pretty easy not to get one of the monsters, just look them up before, and if in doubt dont buy. Thats my advice anyway.