The ID field is best left alone, here is some info that will help explain that and Track-It! Inventory in general:
The custom text fields in an Asset cannot be changed to a drop-down.
You can't hide fields in Track-It! Web, but you can edit the field names, and some other attributes of many fields in Technician Client by placing your cursor in the field, and then pressing Ctrl-F2 of Shift-F2 on your keyboard. These changes will also take effect in Track-It! Web.
Please let us know if this helps.