WSell, let's see. I've had plants with Plecos before and had no problems, others have come back and said their pleco tore up everything so, depends on the fish. I still think Silk plants would be okay, just place them well in the gravel, or have them on the edges of rocks and driftwood.
As for treating things you pull out of a river, some folks will say it's dangerous no matter what. I say bleach, boiling and baking will kill anything. It's your call. Most folks, when they find aquarium material out in nature will soak it in bleach and water for a while. I'd go 24 hours, or at least over night. Then, I'd bring it inside, rinse it in some dechlorinator and water to remove the chlorine bleach. After that, I'd boil the stuff for 30 minutes to an hour. If it's big, you can try boiling half at a time. If it's still too big, pop the thing in a 375 degree oven for an hour or so. After that, let it cool., give it a rinse and put it in your tank. Voila. HTH.