i am inclined to agree with Miss Wiggle here.
a lot of LFSs preach incorrect information to get your cash. not all LFSs, just some.
common plecs have the potential to reach between 14" and 24", they are monster fish and need at least 100 gallons once full grown.
do you know if your goldfish are fancies or common? fancies will reach between 10" and 12" full grown, and commons will be 12" and possibly larger, of course only if kept in the correct conditions.
the problem with both of these fish is the sheer amount of waste they produce, they are both extreme poop machines! how long have you had these guys together? i've heard of plecs suffering from being on the bottom of tanks with large amounts of poop, poop will cause ammonia problems within the tank which then rolls into a snowball of problems which can result in fish fatalities.
as for temperatures, common plecs, as with the majority of plecs, are tropical fish needing warmer waters, if goldfish are kept in tropical tanks their metabolism speeds up, making them poop even more! (lots of poop=bad) and other health problems.
the general goldfish tank size rule is 1 goldfish for 20 US gallons, and 10 US gallons per goldfish thereafter. However if they're common goldfish (comets etc) they're best suited to pond life rather than tank life.
dwarf gouramis have been getting bad press lately. there are several gourami specific diseases circulating, and over breeding has caused them to be very delicate, much like guppies and neon tetras.
i know you may have become attached to your fish now, but you really have to consider whats best for them in the future and not just right now because they seem ok. fish grow a lot faster than often anticipated. they may only be "ok" for another month or so.
rehoming the common plec whilst he's still small is your best option, once they get over 6" it's very hard to rehome them, theres an influx of large common plecs out there that have outgrown their homes. unless of course you can plan a 100 gallon tropical home for him!
as for the goldies, tankmates are notoriously hard to find, they're curious creatures, and if the tankmate is smaller than their mouth, they will try and eat it. if you've grown attached to a specific one, you could rehome the other 3 and let him have the tank to himself for a while, and consider an upgrade to a 30-40 gallon for him further down the line when he starts to get monstrous (with poop and an appeitite to match).
