InfoPath Dev

I have a field on the form that is prefilled with prefix characters based upon a choice field elsewhere in the form. The requester is to enter in subsequent characters however we do not want them to overwrite the prefix characters. How can I block the prefix characters?

I also would like to ensure that the field must contain 5 or more characters (including the prefix). How would I write that as a rule? 

 You could use a 2nd field and display the choice as a calculate value.

If uses can't change the original choice field you'll need to write a rule perhaps in a section to hide the original choice control and only show the chosen value.

As for your rule.  I quickly put together the following. I'm sure you might be able to do it quicker too.

Field1 - your character text - on your form

Result (hidden) - with the default value of sting-length(field1)

Warning Text field - only to appear if there are too many characters in field 1 - use section to show or hide.

On the Result field I used the following formula Result>5 set field value of warning text to too many characters.