I try to answer each question separately...
1. should i maybe start and use all of them? then wipe all the drives and start from scratch and then configure for the next one? that way i get experience with all the different configurations?
Not sure what you mean. Test all the different distributions? Or test all the different servers (services)? That would be a very nice research, but very time consuming as you are a Linux starter.
I would suggest that you select one distribution for your server and play with that. You mentioned that you have 4 systems: 1 server and 3 clients.
Example setup: Fedora 12 on the server. For the clients you can use Windows XP, Windows 7 and Ubuntu 9. That would give you a very nice test network and you can learn the differences between Fedora & Ubuntu.2. can you install CentOS on a normal desktop and use it as a client? like ubuntu? you said you use fedora, so it sounds like your saying you can use any distro and use it as a server then? or is this a wrong assumption?
Linux comes in many flavours. Basically you have 3 main families: RedHat, Debian & Slackware. They all started somewhere in 1993. The rest of the distributions you find are derivatives.
Check out: http://futurist.se/gldt/gldt93.png
The main differences between the 3 families is the package management.RedHat: rpm-based systems, like Fedora, CentOS, ...
Debian: deb-based systems, like Ubuntu, Mint, ...
Slackware: tar-based systems, like Suse
These packages are not compatible, so if you choose a package type, you stick with it on that system.
The Slackware distro's (tar-based) do not have dependency checks; you have to do that manually. I would not recommend that for a starter.
Further, if you stay with big distro's as Fedora , CentOS, Ubuntu, ... yes, they can be used for a client and a server. The repositories offer several server software. In fact, you can make a server of any system. It is just a matter of installing the software and configuring it. The core system (kernel, ...) is the same for a desktop or server system.
3.can you have all these different types of services on one server? in the link you gave me it looks like that is exactly what the guy is doing. is it a good idea in the industry?
You can have all the different services on one system, yes. For learning, this is no problem. When you setup a server for professional use (industry use), it is possible, but not wanted due to performance and security issues. But then virtualisation might be a solution.
For my Linux courses, I have the students to setup several services on one system too (dhcp, dns, mail, file sharing, ssh, database, web, ftp, ...). During their project weeks, they split the services between different machines, but that is just a matter of availability of hardware. For learning purposes, this is not a requirement.can you suggest maybe any way i can start this? i have no idea where to start or what to do. should i start with CentOS or fedora? i don't have a personal taste coz i havent worked with anything yet. do you have any suggestions on how to start this training setup of mine?
Just pick one: Fedora or Ubuntu. For these distro's, you will find a lot of information on the internet. Both have their forums. For specific Fedora problems I use http://forums.fedoraforum.org/
Suggestion: start with Fedora and use the link I put in the previous post. Follow the instructions and look how far you get. Perhaps you have to start over at some time, but that is just part of the game. In my early Linux days (1998), I messed up a lot of test systems... now I am a full-time Linux user (home & work) and I am a Linux teacher...
If you don't do, you won't make mistakes...
If you don't make mistakes, you won't learn...
So, just do... and learn...