Plecs yes - but not all of them. Plecs vary greatly and some won't do well in teh conditions discus like (namely the rather high temperatures). Equaly, some plecs, it's been suggested, will suck on the discus's slime coat - I haven't ever seen this happen nor found anyone so far who has had it happen - but it's worth noting anyway.
With tetras, it depends on which species. Small ones will get eaten but neons shouldn't be kept with discus anyway because they can't deal with high temperatures. the typical tetra discus tankmate is the cardinal - just search for discus tank pics and you'll almost inevitably run into cardinals Even cardinals can become lunch for larger discus if they are not fully grown though.
With the sharks - the obvious answer is 'no'. However, if you class siamese algae eaters and oblongus as sharks, then it could be a yes. Many people keep these fish (siamese algae eaters do best in schools) with discus as algae-control.
I'd ask about discus tankmates in the new world cichlids section (as well as looking in the fish index as you've already been told). You should also be prepaired to do a lot of independent research if you're considering taking on these fish.