Can someone sort out my confusion?

I have an old case that was micro atx sized and I wanted to make into a server but I have one guy telling me I can't use micro atx because it will fry itself and fall over but another guy saying its nothing more than a smaller atx board which most people use for a server so it would work just as well. Also it was going to be for a minecraft server.

With the right cooling it will be fine. Most nas servers are based on boards like micro atx.

Of course you can d it. I have machines running as Linux based servers doing file serving, web serving, multiple mail serving and even being used to run workshops as well, all based on standard pcs, many of them have been running for 7 years or more, and All those older ones were used because they were no longer capable of running the then current versions of Windows. And if you look at many servers, they do use smaller than standard boards so they can be rack mounted. So the first guy is an idiot. Tell him until he has learned ANYTHING about computers he should not give advice.