Hi and welcome to the forum
Watch the rainbow sharks. They are territorial and can become aggressive when they mature. Normally you keep 1 per tank to stop them killing each other.
Are you planning on getting a bigger tank in the future?
I ask because discus can reach 10 inches in diameter and adult discus need big tanks. The minimum sized tank for a pair of adult discus is 4 foot long x 2 foot wide x 2 foot high. Having more than 2 fish means a bigger tank like a 6x2x2ft.
Do you have many live plants in the tank?
If not, you should add some. Discus do a lot better in planted tanks. The following link has some information on discus that might interest you.