With regard to your Performa, try zapping the PRAM at startup: shutdown the computer, start up, and hold down the key combination Command-Option-P-R until you hear the startup chime two or three times. PRAM holds various parameters associated with your Mac OS, including the time/date. If you plan on keeping it, I'd also replace the CMOS battery on the motherboard. If you google "cmos battery performa 631" you'll find a whole lot of sources for the battery.
With regard to the Centris and 7600, I'd first try the PRAM zapping trick with them. Your monitor info is also stored in PRAM. If that works, it might be worthwhile replacing the CMOS battery for those computers, too. It all depends on what recovering your data is worth to you.
You could create an AppleTalk network by cabling the Macs together if they all had Ethernet ports but that's not an option for you. Assuming you can get the 7600 and Centris up & running, I'd go through the files and identify what you absolutely need to keep, and compress the ones you're going to keep to reduce their size. Then I'd do one of the following (assumes you're wanting to keep the Performa):
1. Create an online file storage account; here's a list of
28 alternatives. Note that the amount of free storage you're going to get is limited, so you'll have to transfer your files in "chunks". Dump your files from the 7600 and Centris, and download them to your Performa. When you're done recycle your old Macs or sell them for a few dollars/euros on a site like eBay or craigslist, or in your local shopping newspaper.
2. Alternatively, buy a SCSI external hard drive large enough to hold all the files from the old Macs. Dump all the files to the hard drive as above.
A great resource for owners of vintage Macs is
Low End Mac; they have the specs for all Macs, sources for parts & purchase, mail lists, etc.
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