Yes, it is appropriate to reach out the plaintiff. I would do it in writing. And if they don't respond appropriately, I would keep a record of their response or non-response.
It is quite "creative" to use small claims court to collect on copyright infringement issues. Very low bar of entry. Judges are typically not knowledgeable on copyright matters. People defending themselves in small claims court have the burden of explaining the judge what this racket is all about.
I advised a defendant to print out several ELI posts, prepare a presentation, and submit it as part of their defense that the plaintiff is inappropriately and under false pretense using small claims court to resolve a copyright infringement matter.
Should I snail mail him requesting he tell me what photos as I am not aware of stealing his photos? I paid hefty to Getty a while back and learned my lesson so removed anything I thought I may not have purchased and now only use 123rf.com.
Is contacting the plaintiff appropriate to get more information?
The kicker here is that the plaintiff is not suing in small claims for anything copyright related, as that is a federal gig, they are dragging people into small claims, stating that the defendant used / misused the subscription service offered without paying the subscription fee... Their claim is likely one of: these im ages are only available through our subscription service, so defendant must have have figured out a way to get to the images without paying.. Judges need to know that these images were once available on multiple sites for purchase and download, and have been removed along with purchase histories..
Playing it this way saves the cost of filing a federal suit, hiring a copyright attorney, etc, etc...