Can you control all your Aniimo in Interlink?

Your other three Aniimo still count, but only through their core skill. In Interlink the second to fourth slots cannot use their second skill, and only the main Aniimo's ultimate is available, so an assist like Somni Wing brings its core skill and never its ultimate.

Assists are never on the field, so every hit lands on your main. Pick your main to survive the whole fight, because it is the only one taking damage and the only one you steer.

Egg Heist works differently. There you switch to your three benched Aniimo to use their core skills, so a team built for Egg Heist does not carry over to Interlink unchanged.

What makes a good assist

Because an assist offers only its core skill, judge each one by that skill alone. Turbo's core skill raises team damage and break efficiency by 15%. Spark Elf's Inspiration raises the team's fire damage by 16%, which suits a fire main. Gracewing's assist skill places healing fields that heal in an area, and that is the only way to keep a main alive when nothing else can take a hit for it.

Some kits also reward sitting out. Turbo builds Shrouded in Mist stacks passively while off the field and reaches it after 20 seconds, which is where an assist spends the whole match.