Initially, I thought that a slann could not. But in 8th edition, the slann is classified as infantry. He can.
This is a very debatable question. Ask your local gaming group what they think. It can go either way.
Well GW has FAQ'd this so the Slann can NOT cast Transformation of Kadon. Which I think is the beast transformation spell but I have no book here so can't be sure. GW has updated the FAQs with magenta wording showing the changes. This also means no flying carpet.
You would think it means no flying carpet, but they didn't specify why the Slann can't cast the spell. They could have just said, 'The Slann is infantry, but not counted as "on foot"'. Would have nipped further rules queries in the bud. I definitely won't plan on being able to have the flying Slann, but some people are still going to try.
Ya I agree they should have just said he doesn't count as on foot. But this implies the same thing, unfortunately people will still try and take advantage of GW not being specific.