Q: I need tips for working with a many Macs

If each computer can connect to the Internet without re-cabling, you already have all the required infrastructure in place. And since these are Premium-quality Macintosh computers, you already have all the software installed. Now just configure File Sharing:

Mac Basics: File Sharing

Significant use of File Sharing requires Gigabit Ethernet Switches and cables to get "near hard Drive" speeds. Anything less is S-l-o-w. Gigabit Ethernet Routers are readily available and not particularly expensive, but they may not be on the "hot products" shelf at Best Buy. A Gigabit Router is not required if you use a Gigabit Switch because local File transfers do not use Router functions.

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Time Machine works in 10.5.8 and later. It is a complex subject, but you CAN back up multiple Macs onto one Time capsule. I suggest you will want to BlackList Mac OS X and most Applications, or the size of the Backups will quickly overwhelm your shared storage device. The initial backup is still so large it still may take overnight for each Mac, especially over WiFi.

Mac Basics: Time Machine Backs up your Mac

Pondini: Apple OS X and Time Machine Tips

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Drives are really, really cheap by historical standards. Buy several spares and several External enclosures.

Yes, you can re-format and re-use older SATA drives in newer Macs -- this will save you UNDER US$100.

You can collect all the Shared files onto a Mac running Mac OS X Server. However, the learning curve has been daunting, and Server software is not a requirement for what you are doing.

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You would be better served by limiting your queries to one topic. Readers with deep expertise in that specific topic are more likely to contribute that way.

1. Networking: IF this is a home network or one in which you have trusted users, basic networking is easy; just connect the systems to a switch (or more likely a series of switches) and enable file sharing on all the systems. I've had a network like this for six macs in my house. If you want wide-open access to all machines, log onto the other machines as that machine;s user. i.e, log ongo the iMac G5 using that system's name and password. That gets you access to everything. You can simplify this by using a common password for all systems.

This technique is TOTALLY insecure, so only use it if all users are trusted (all mine are family) and no important personal or financial info is involved.  In a less trusted environment you can use the same layout but fileshare as guests.  In either case one switch connects to a router for internet access. Anything less that gigabit (1000 baseT) switches will result in very slow file transfers. The router can be 100 baseT since your internet connection will be slower. (If it wasn't you'd be asking your IT manager instead of me...)

2. Hard drive issues: Yes, you can "re-purpose" the hard drives as long as the drive itself is compatible with the new system. A PowerMac G5 uses SATA hard drives, same as the Mac Pros. Do the re-formatting on the receiving system to make sure the OS likes it. The norm for data drives is Mac OS Extended, for system drives use Mac OS Extended (journaled).

3. I've got nothing on Time Machine...  sorry