I've split your post here because there maybe more information that you need before making that decision.
In any case, did you try searching for pictures in our Aquarium Photo Gallery? If you did, you may have come across this,
Crossocheilus siamensis (courtesy of Justikanz)
Otocinclus spp. (courtesy of rwalker)
FYI, most Otocinclus are referred to as O. affinis although they aren't necessarily O. affinis due to misidentified pictures on the Internet. For your purpose, i.e. to clear uneaten food or clean some algae, any Otocinclus would do.
However, the bare tank is of some concern. Otocinclus don't seem to do as well in bare tanks compared with planted tanks. Having plants seem to much better for the well being of the Otocinclus.
Also, you should know that discus are known to excrete protein rich slime from their body, especially during spawning period. The slime is likely to be easy food to the Otocinclus, hence they may take to sucking it off the side of your discus. This in turn may irritate the hell out of your discus.
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