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Usally when people are talking about adapting SCA drives for use in non-SCA machines, they're talking about doing it in order to save money, so they're using the cheapest SCA->Whatever adaptor they can find... which results in a lot of heartache for people trying to adapt from SCA.
50-pin drives are self explainatory. These have some form of Narrow SCSI which the Mac will like naturally.
68-pin drives are some form of Wide SCSI, like Wide, Fast/Wide, UltraWide, Ultra2Wide, U160, or U320. These drives use a 16-bit data bus to transfer half a word in each clock cycle, as opposed to the 8-bit bus that Narrow SCSI uses. Almost all (I believe you have to support this in order to support the full SCSI spec, actually) wide drives can be used on Narrow SCSI busses by terminating the upper byte. The drive can sense this and will go into a narrow-scsi compatibility mode. It usually works to just leave the upper byte floating, and that's how really cheap 68->50 pin adaptors work, but it's not particularly reliable, especially if you're using more than one 68->50 pin adaptor on your SCSI chain.
80-pin SCSI isn't actually a SCSI flavor at all, it's really just a format that SCSI drives can come in. It stands for Sun Connector Architechture and is why most RAID cabinets and also the internal drives in Sun computers don't need any configuration: Those 80 pins provide not only 68-pin SCSI, but also drive power and most of the necessary jumpers, especially the SCSI ID jumpers. That way, you can just take a couple dozen SCA drives, slap 'em into place in your big RAID tower, and be chugging along without spending an hour making sure you don't have a SCSI ID conflict. :-)
80-pin drives aren't required to be 68-pin SCSI, IIRC, so a rare SCA drive that doesn't support Wide might exist. All the same cautions apply to attaching an SCA drive to a 50-pin bus as attaching a 68-pin drive, except that it's even more common for cheap adaptors to cause heartache to those trying to save money.
Go ahead and put those SCA drives into your 68k, but make sure you've got a fair bit of time laid out for getting them up and running. Hopefully it'll just work out fine, but you might have to debug something, possibly including replacing some of your adaptors if they're not working well.
Any 68k Mac will support as large a disk as you care to put into it, though the OS you are using will decide what exactly you can DO with that disk space. System Software older than 7.5.5 is limited to volumes of 2GB, so an 18 GB drive would show up as nine volumes on your System 6 desktop! System 7.5.5 is limited to a partition size of 4GB, so an 18GB drive shows up as 4 volumes and change on your desktop. System 7.6 supposedly has larger volume support, but I haven't seen it help any. MacOS 8.1 DOES support very large volumes--how large I'm not sure, but with HFS+ support in 8.1, I'd be interested to try multiple-hundred GB drives just for kicks!
One big problem you'll run into with modern SCSI disks is lack of support for formatting them. If you're running System Software 7.5.5 or older, instructions for patching Apple HD SC Setup are available here. For users of System 7.6 or newer who need to use Drive Setup, instructions for patching Drive Setup are available here. By applying these patches, the Apple drive formatting tools can be used with nearly any drive. In addition to being free, the Apple tools have the advantage of not being nearly as unstable and conflict-prone as common 3rd party tools.
Another thing to note is that all modern SCSI drives lack support for termination. Not all 68k's SCSI cables have a terminator at the end, but this is important. If the cable doesn't have a built-in terminator, make sure to include at least one native 50-pin drive at one end to provide termination, or attach a SCSI terminator there.
To really get your mileage out of the drive, get a SiliconExpress IV or JackHammer NuBus card to get a 20MB/s Fast/Wide SCSI bus in that 68k--the disk will be able to start to hit its stride on such a fast bus.
Text and Images copyright 2003-2008 Tyler Sable