Below reply to your queries.
Any suggestions in how to prevent users from crashing the system using the Planning connection?
add these changes in Windows registry so that it will have netretry count increased that will stop from crashing.
Can one limit the number of rows the same as the Essbase connection?
By default its 50000 in essbase but again as planning data resides on essbase you cannot but yes you can change the default value from 50K to less as well in essbase cfg file.