clown loaches are meant to be in groups of at least 3. Your tanks are too small for them really and they will need to move to a larger tank. They are not good tank mates for oscars, as they are peaceful fish and oscars aren't. Your tanks are way too small for an oscar who will need a much much larger tank and some extremely hardy tank mates (if any) definately not clown loaches.
You must be aware that your Oscar will outgrow a 20 gallon tank within 2 months, and you will then need a 75 gallon minimum for a pair, preferably 110 gallons. As it is if you bought 2 juveniles you don't know if it will work out, it's best to purchase 3 or 4 and let a pair form naturally.
Agreed, except that his Oscars are already in a 20 gallon
As for the pairing, you are right, nothing is sure, but bringing them up and allowing them to form their own bonds increases the chances of success with it.
Every fish shop that I know of would take back the extra fish for store credit.