Powder blues stay about 2 inches... and can be in as little as 10 gallons. However, the Electric Yellow Labs (which can be a little aggressive) can reach 5 inches. The golden mbuna is another aggressive fish, which gets to about 4 inches. Basically, you're heading into the direction of a tank that should be set up with typical Lake Malawi conditions.
The p. blue gourami doesn't match well with these fish, in my opinion.