If you are using the paste (that's what I use) give the serama rooster a BB sized piece on the side of it's beak, hopefully he'll eat it. If not,try a small piece of bread with the BB sized paste on it. Ivermectin pour on will kill gapeworm also. You'd use a syringe with a needle to precisely put 2 drops of the ivermectin pour on directly onto the skin on the back of the neck, it'll be absorbed through the skin, avoid getting it on any feathers. I use ivermectin pour on also for my standard size hens and of course they get more drops from the needle.
I'd like to make a correction, Safeguard paste will NOT kill gapeworm. Buy the 10% suspension from TSC and use 2.5cc/ gallon of drinking water. That WILL kill gapeworm. chickenzoo is also correct...ivomec will kill them too.