Brittle star as in little white guy or as in Green Serpent start? The green guy is great at eating excess food waste but in time will grow large enough to go after your fish.
What you added would not do much in a tank that large and I do not see why you can not add fish if the cycle is complete. I like the idea of adding fish slowly but you need to add them in the correct order. That is tough to do unless you have your "Fish List" done. You do not "have" to make a fish list but it does a couple things. It allows you to order the addition, shop for the right animals, discourages impulse buys.
At any rate congrats!! adding the first fish is a fun moment and especially in a tank that large, you feel good about getting him out of that tiny display and into a tank with some elbow room.