I did the path Ag. Ioannis- Selouda-Ag. Roumeli some years ago, in both directions. It's a very easy path, very nice to walk, beautiful landscape. I only recommend you to have enough water to drink and avoid the hottest hours of the day. From my point of view a guide isn't necessary: the parth is very well marked, in good conditions, not dangerous. But this is obviously only my opinion.
In Ag. Pavlos there is a taverna were you can possibly eat and drink. Then one other hour and half to get Ag. Roumeli.
Be careful just before getting Roumeli: the path leaves the coast and climb a little: here there is a little slippery and narrow section of path, above rocks, all covered by sand. But it only requires attention.
Have a nice trip!
Marco from Italy