For example 5.6.6 Release notes say:
Product Integration and Support:Fortinet Single Sign-On-5.0 build 0271 and later
FSSO is: Collector Agent + DC Agent + Terminal Server Agent
Therefore you should be upgrading all of it.
"I couldn't find if the 220.127.116.11 DC agent would work with a 18.104.22.1681 collector
It very likely will, there are usually no major changes that would prevent communication between them two.
In case of any issues however, support might ask you to upgrade to supported version.