Rainbows

I love rainbows. I currently have Boesemanis, Praecox, Threadfins, Madagascans, Celebes, and Turquoise. The males are generally more colorful than the females and show their best coloration when they want to spawn (which is why it is advisable to get females, too, not just males). IMO, the Boesemanis have the best color. Keep in mind that their color also comes with age--when they are younger, they don't display their best colorations. Rainbows are great!

They should get along with your other fish, but I might question the gourami--what kind is it? I keep some of my rainbows with three honey dwarf gouramis and have no problem. I also have neon tetras, rummynose tetras, otos, cory cats, and plecos.

Right now I have 3 small ornate rainbows in with gouramies (blue/gold/opaline) and don't have a problem. I also keep australian rainbows (quite large) in with small tetras, rasboras, dwarf gourami and others. They are great looking and don't bother other species.

they like hard water which is a plus for us around here! you should look into the fork tails, so nice!

Here are a few Rainbows
Boesemanis

Banded Rainbowfrom the Goyder River

New Guinea Red Rainbows

I personally don't like the New Guinea rainbow.

As for hard water, wouldn't your local fish store be keeping them in the same water as your tank? I have mine in water that is 7.6 pH. My guess is they are used to whatever you have already because the store probably uses the same.