On a custom agent, yes: click Edit Skills on its page to add or remove integrations. The price changes right away, prorated for the rest of the billing period, the credit pool adjusts, and the agent restarts.
Marketplace agents keep their built-in skill set: "Cannot edit skills on a template agent — only custom-built agents support skill editing." You can still change their preferences, scheduled tasks and settings. To choose integrations yourself, build a custom agent.
Some skills need others: "Cannot apply: some skills require other skills to be enabled."
New credentials: a new skill may need its card filled in, so the agent can show Setup Required (see Why won't my agent start?).
Details: SkillsApply.