Currently I use Skritter for iOS only to study traditional characters. I would like also to practice reading (not writing) certain individual simplified characters (not multi-character words). How can I do this with the iOS app?

Skritter doesn't have this as a feature, but I think you can achieve this with a trick (I didn't test it, so give it a try).
1. Set your account to add manually only, you need to have full control during loading of new items.
2. Traditional/Writing: Set your account to traditional and to writing (the account, not the list). Now load your new items and learn through at least once.
3. Simplified/ Reading: Do similar with this, account to simplified and reading, then load.
4. Review: Set your account to traditional and simplified, writing and reading. Don't load any new items in this mode.