A change in diet and provided ad lib at the same time is why they are runny, they also smell more because there is more undigested stuff in the poo.
Iams is a poorer quality food compared to RC or JWB, a lot more misc' grain.
I cant see that a sphynx fed 3 times a day will get cold and ill, he gets iams and the others back to their normal routine, it obviously worked for them. Having 5 cats healthy and just the 1 problem cat has to be better than making 5 ill, long term diarrhoea is not good.
Hels has a Sphynx called Bonnie, maybe she will see this and offer her expertise.
I also agree with Elas sentiments, the original cats should come first.