Two small form alterations

I have a three level taxonomy category system that goes Parent --Child ---Grandchild When the user posts form 1: -The GRANDCHILD category is posted with the form. I need a small alteration done on this [login to view URL] is a textbox called keywords.I need this textbox hidden and the automatically populated with the grandchild category that is posted with the [login to view URL] be a very quick and easy task to [login to view URL] must only be one category out into the textbox when the form is submitted. Form 2: This also has a textbox called keywords. However,this time when the user types in the keywords textbox,an autocomplete will come up with only the grandchild categories from a custom [login to view URL] the user selects a category from the autocomplete,that category will be put into the keywords textbox. The user should be able to put up to 7 grandchild categories in the textbox which all seperate by a comma. If the user has selected more than 7 categories,then the form should tell the user that they can only have a maximum of 7 categories. Please use jquery in no conflict mode as it will otherwise interfere with my [login to view URL] should be a pretty easy and straight forward job for someone who knows what they are doing.