BuddyPress pages are BuddyPress template driven, most of the themes are compatible with BuddyPress by default same goes for Divi theme. All the BuddyPress pages like Member directory, Group directory, single-member, or group page will show up as default BuddyPress template as Divi does not override these templates. You will need to customize these pages to match your site style.
If you are starting a new website and social community feature is the most important one, I would recommend go with a BuddyPress Compatible Theme to save time and cost. Here is a free BuddyPress Theme you can see he BuddyPress templates are modified.
For upstream plugin, it needs to be tested for any possible conflict, ideally, it should work together.