Our extensions do not have built in support for sh404SEF, and we can't advise on the use of sh404SEF because we don't know anything about it.
A user contributed this solution to using FlexiContact with sh404SEF. Other users have confirmed that this works:
Lookup the Flexicontact URL and ID in the Joomla menu manager. My URL was "index.php?option=com_flexicontact"
Open the sh404SEF URL manager and create a new URL.
Enter your desired SEF URL, and for the non-SEF URL write the URL from step 1 but including the menu item ID number. For example index.php?option=com_flexicontact&Itemid=132
Save. Note that sh404SEF will rewrite the non-SEF URL to include the language "index.php?option=com_flexicontact&Itemid=188&lang=en
This information may or may not be helpful for our other extensions.
Several users have indicated that they would like to use sh404SEF to add meta keywords to specific pages in order to improve their search rankings. Please not that this is a misconception, as described in this Google blog: