Totally agree, quarantine is super important! Need to protect your existing stock from potential diseases from new comers.I'd wait until you set up the larger tank for more fish...& then add a pearl gourami & more of current your fish, they'd all like more friends (like others have said.
Use your 20g as a quarantine tank for each species. I QT for 4 weeks of health. I can't stress the value of QT enough...but see what's going on with your faded kuhli first.