Personally I recommend that you install BPS Pro first because website security should be added to a website as soon as possible. You can install BPS Pro at any time. Add the BuddyPress wp-admin skip/bypass rule in the link below in BPS Custom Code. S2Member login / login security overrides BPS Login Security. See the S2Member link below.
BuddyPress admin-ajax.php and post.php skip/bypass rule: http://forum.ait-pro.com/forums/topic/bulletproof-security-buddypress-compose-message-autocomplete/#post-1588
S2Member known issues: http://forum.ait-pro.com/forums/topic/bulletproof-conflict-with-s2member/#post-16727