I wrote this a few years back. if you have questions after you read it, please ask.
First you need to know what the "medicated" is in the medicated feed. It should be on the label. Usually it is Amprolium, Amprol, some such product, but until you read the label, you really don't know. Most "medicated' feed from major brands for chicks that will be layers uses Amprolium, but there are a few out there mostly for broilers, that use other medicines. I'll assume yours is an Amprolium product, but if it is not, then realize everything I say about it may not apply. And it is possible that the "medicated" is Amprolium AND something else.
Amprolium is not an antibiotic. It does not kill anything. It inhibits the protozoa that cause coccidiosis (often called Cocci on this forum) from multiplying in the chicken's system. It does not prevent the protozoa from multiplying; it just slows that multiplication down. There are several different strains of protozoa that can cause Cocci, some more severe than others. Chickens can develop immunity to a specific strain of the protozoa, but that does not give them immunity to all protozoa that cause Cocci.
It is not a big deal for the chicken’s intestines to contain some of the protozoa that cause Cocci. The problem comes in when the number of those protozoa gets huge. The protozoa can multiply in the chicken’s intestines but also in wet manure. For them to reproduce they need some moisture. Slightly damp isn't an issue, soaking wet is. Different protozoa strains have different strengths, but for almost all cases, if you keep the brooder dry, you will not have a problem.
To develop immunity to a specific strain, that protozoa needs to be in the chicks intestines for two or three weeks. The normal sequence is that a chick has the protozoa. It poops and some of the cysts that develop the protozoa come out in the poop. If the poop is slightly damp, those cysts develop and will then develop in the chick's intestines when the chicks eat that poop. This cycle needs go on for a few weeks so all chicks are exposed and they are exposed long enough to develop immunity. A couple of important points here. You do need to watch them to see if they are getting sick. And the key is to keep the brooder dry yet allow some of the poop to stay damp. Not soaking wet, just barely damp. Wet poop can lead to serious problems.
What sometimes happens is that people keep chicks in a brooder and feed them medicated feed while they are in the brooder. Those chicks are never exposed to the Cocci protozoa that lives in the dirt in their run, so they never develop the immunity to it. Then, they are switched to non-medicated feed and put on the ground where they are for the first time exposed to the protozoa. They do not have immunity, they do not have the protection of the medicated feed, so they get sick. Feeding medicated feed while in the brooder was a complete waste.
I do not feed medicated feed. I keep the brooder dry to not allow the protozoa to breed uncontrollably. The third day that they are in the brooder, I take a scoop of dirt from the run and feed it to them so I can introduce the protozoa and they can develop the immunity they need to the strain they need to develop an immunity to. Since I keep my brooder extremely dry and the water clean the protozoa can't reproduce so every three days I give them more dirt from the run so they get more protozoa and can develop immunity. I don't lose chicks to Cocci when they hit the ground.
There is absolutely nothing wrong with feeding medicated feed to chicks, whether the protozoa are present or not. It will not hurt them. They can still develop the immunity they need. But unless the protozoa are present, it also does no good.
If you get your chicks vaccinated for Cocci, do not feed medicated feed. It can negate the vaccinations.
The way they make Mash is to gather all of the ingredients and grind them up to make a powder. The way they make pellets is to dampen Mash, extrude it through a dye, flash dry it, and break it off into pellet length. To make Crumbles they crush the pellets a little to get to crumble form. As long as the analysis and ingredients list are the same, there is no nutritional difference.
One problem with Mash is that the ingredients in powdered form can segregate by specific gravity. The heavier ingredients sink while the lighter ones rise. They may not get a balanced diet. The way you get around that is to make a paste out of the Mash while it is still mixed up. That's why I prefer Crumbles. Less work.
As far as less mess, that depends a lot on how you feed it. They can sweep food of any kind out when eating. Much worse, if they can stand in it they scratch. It's usually less mess and waste if the feeder does not allow them to stand in it and has smaller compartments so they cannot sweep their beaks.
Sometimes it helps to have their feeder on a tray that collects any feed that spills. They might still eat it as long as it is not lost in the ground or coop floor.
