I did a fish in cycle with my African Cichlids. Now my Cichlids are very hardy fish to begin with, I wouldn't even think about trying it with any fish that was even a little sensitive. That said, the only reason my fish never showed signs of stress is that I was very OCD about the whole thing.
With a Master test kit I tested the water every morning and every evening. If at any time the ammonia was .25ppm or higher (higher rarely happened) I would do a PWC on what ever amount it would take to get it back to 0ppm. Sometimes, it took doing 75%PWC twice a day. But there was never even a slight chance that my fish would get ammonia or nitrIte poisoning or burns.
They truly were just fine and didn't suffer during this time. But I did, because I kept the ammonia so low and then the nitrItes low, the cycle took two and a half months. That's two & a half months of twice a day testing and twice a day PWC. And I used Prime for the water and Big Al's conditioner with Aloe to keep the fish happy and healthy and slimy.
My life revolved around that tank for nearly 1/4 of a year.
But that's what it takes to do a fish in cycle and not loose or hurt any fish.
I now have a 5g bucket with a cheap heater and a cheap small filter and all it does is grow nitrAtes. I have extra gravel and filter media inside the bucket so that if I ever need to grow nitrAtes in another tank fast. I can do it within a week.
Since there are no fish in the bucket, I let the NitrAtes grow all the way up to 160ppm. I only do PWC in the bucket if it starts to smell, as bacteria can. With the nitrAtes that high in the gravel and extra filter media, it transfers and grows in a new set up so quickly that I was able to cycle a 55g tank in 5 days.
It is an easy, safe way to seed your own tanks. You never need to barrow from anyone else, or worry about whether or not the nitrAte seed is disease free, it's yours, you know it's safe.
With a 5 day cycle it doesn't matter if you have fish in or fish out.
I keep the bucket under one of our larger tanks, no one even knows it's there. If I need to cycle a tank in an emergency I can.