Hi
I haven't seen a form with a hierarchical species selection but I shouldn't think it would be too hard. You would probably just be looking at a little bit of JavaScript (in a JQuery style), modifying the query that the autocomplete executes based upon the primary selection.
You might choose to have a separate species list for each group or use a single species list and filter on the taxon_group field. The latter might lead you in to using non-standard taxon groups though and there may be some advantage in adhering to standard NBN taxon groups (should you be in the UK).
The template for the JavaScript that the autocomplete control executes can be found in the indicia_templates array at the top of the helper_base.php file.
You'd probably want to make the primary selection a custom occurrence attribute so that the value is stored and can be reloaded when editing records. You could maintain a termlist of your higher level groups or perhaps build the list by a query on the names of your taxon lists.
Take a look at autocomplete() and select() in the API documentation
It is certainly possible for autocomplete to work on both common and scientific names. iRecord demonstrates this. It works by virtue of searching the cache_taxon_searchterm table. This even supports an abbreviated 2-3 character lookup on scientific names e.g. haaxy finds harmonia axyridis.
Jim Bacon.