Yes i add some plants in my refug that was mailed to me, more snails, and hermits and 2 fish.. tank has been up and running for about 2 months with animals in it and about 1 and half before that. No that i think about it i probably getting some die off from those plants.
Do the plants look pretty health? I would leave them in if they seems healthy. Remove any that are clearly dying. The health plants will help with the nitrates at least. I am guessing you are experiencing a mini cycle due to increase in bio-load. I would due a water change daily, turn up your skimmer and back off on the feedings. Getting the ammonia and nitraites down to zero is very important. This will happen as the cycle completes and will be helped along by the water changes. I would also raise the alkalinity to stabilize the pH swings. Baking Soda will work. Here is a good calculator.