If it was my tank and I was going to run either the outlet or intake along the bottom, under a sand substrate to the other end, I would probably get some aquarium safe silicone, smear it all over the plumbing and then roll it in sand (the same sand I'd be using for my substrate).
That way when the plumbing is eventually exposed (or partially exposed) by digging cichlids, it wouldn't stick out like a sore thumb.
Lastly, you mentioned you may want to do a pair of Salvini's or Severums - if it were me I would lean towards Severums. A pair of Salvini could quite possibly kill your oscar if they ended up being a true pair and spawning. If you go with Sevs I would get 6 or so to start, let them pair up naturally and grow to about 5 or 6 inches, and then introduce a single oscar... Oscars grow incredibly fast from what I hear (never had one). I have kept severums (mostly Rotkiels) and they are not what I would consider fast growers.