if its one and a half foot, it could well be another sailfin, or just a common-but hey you may bag yourself a bargain like an adonis or Pseudacanthicus sp. or somethin...
but anyway i wont lie, if its a gibby like your other, there is a high chance that your 7" is going to get bullied by a 18" (thats a fully grown one). unless you have adaquate hiding places and a decent sized tank, id be wary about doing it, but its hard to give advice on what to do. you should check it out though, just cause it might be something cool... you never know
big plecos are territorial plecos with age, but thats not to say it isnt possible to keep a few together, because a lot of people do it (some on here)