I'm glad that you are interested in TeamPulse.
1. About the first question, all values for the items created before the custom field will have NULL value in the database, so you will see them as blank values in the UI. When you open this items you will be able to see and edit the custom field.
2. The custom fields behave exactly like the other fields in TeamPulse. You can edit them and you can add them as column in the grid view. You can also filter and sort by these fields.
3. There is a limitation in the Microsoft SQL Server about the number of columns in one table. You can only create a table with maximum of 1024 columns and this is the maximum number of custom fields that you can add for the whole TeamPulse application.
I hope the information is helpful. If you have any other questions we will be happy to answer them.