I would QT them in a large tupperware tub for a few weeks to see what kinds of things might have hitched a ride. Then possibly give them a dip in a mild bleach or hydrogen peroxide or excel solution before putting them in any of my main tanks.
The plants will get much bigger than a half dollar, I got some water lettuce that sze on ebay for my tank, they look really cool and everyone likes the look of them, your fish will love them too but they are now the size of a tea saucer in diameter, although there may be different strains of the plant. Mine came from some guys pond in new hampshire and I did not quarantine them at all as I am not the most cautious aquarist. The only hitchhiker I got was springtails which i dont mind and my cloud minnows eat enough to keep them in check. the plants do a great job of keeping algea in check but beware they do steal a lot of nutreints from the tank and they reproduce at an alarming rate, i started wih 6 and have 20 two months later. Also although they are only supposed to grow 8" deep, the roots on mine grow very deep, like all the way to the bottom of a 29g tank, so I cut them at around 5-6" which seems fine but makes the roots bunch out. I would say go for it but keep an eye on them and start with one or two and you will have more than you need in no time.cool. I'll definitely QT them, I have enough tanks to go around.
They are all about the size of a half dollar, do they get larger? They probably didn't have a bunch of nutrients in the container pond.