That depends how your SIP Publish Trunk is configured now. If it’s simply pointing at the IM&P publisher (ie no resiliency) then no changes are required to it when adding IM&P subscribers. If you’re using DNS SRV records then you would want to add additional less-preferred records similar to to your existing IM&P subscriber.
Certificates have a similar answer: it depends what you have CA-signed now. Look at Certificate Management under OS Admin and see what’s CA-signed. If you’re using multi-server certificates they will need to be regenerated to include the additional subscribers in the SAN. Also, here’s a useful link:
https://www.cisco.com/c/en/us/support/docs/unified-communications/unified-presence/116917-technote-certificate-00.html
Assuming everything is configured properly and you follow the procedure as laid out in the documentation, yes the new subcluster nodes will join the XMPP Fabric automatically and communicate with the existing ones. Here’s a starting point:
https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/install/12_0_1/cucm_b_installation-guide-cucm-imp-1201/cucm_b_installation-guide-cucm-imp-1201_chapter_0100.html#CUP0_TK_P8901E0C_00
