Hi Suhartono!
Yes, i would recomened keeping everything SSL if you multisite is already running on SSL.
So SSL can be a bit tricky on Cpanel version. The Cloudways servers are able to assign Letencrypt SSL automatically,
In the case of Cpanel, You will have to add manually grant SSL to the new domain you add.
e.g
other domain http://site1.com – Cpanel – Assign via AutoSSL feature in Cpanel
other domain http://site1.com – Cludways – Assign via SSL via cloudways dashboard
In WPultimo setting, you can force all domains to load in SSL, this woud mean you have already installed an SSL.