Black skirt tetras can be a problem if they start harassing the elephant nose and gouramis. If you want more black skirts then try them but watch them closely and if they start nipping the gouramis or elephant nose, you will have to move them out. Other fin nipping tetras to avoid include Serpae and Buenos Aires tetras.
How long, wide and high is your tank? 70 gallons should be fine for a few more dwarf gouramis but it depends on the tank length and width.
Make sure you quarantine any new fish for at least 1 month, especially dwarf gouramis. They carry an iridovirus that is highly contagious and kills pretty much any fish it infects.