The success rate of transferring video from DV tape into iMovie 10 seems to have diminished somewhat with the more recent OS's. There have been some posts on here over the last couple of years where audio is not captured with the video or that neither audio or video is being captured. There doesn't seem to be a particular reason for this as no doubt there are many others who do have success. Also to note, that the Apple help on iMovie 10 states that tape can be captured, so Apple does expect it to work. Just saying that peoples experiences do vary.
If you have access to iMovie version 9 running on El Capitan or earlier then I would say that you would be able to capture the DV tape and use Hi8 via passthrough without problems.
Depending on your Mac, apart from the FireWire 4 pin to 9 pin cable ,you would need up to two more cables if your Mac does not have a FireWire port. A FireWire to Thunderbolt adapter.About $29
https://www.apple.com/uk/shop/product/MD464ZM/A/thunderbolt-to-firewire-adapter
And a Thunderbolt to Thunderbolt 3 (USB-C) adapter. About $49
https://www.apple.com/uk/shop/product/MMEL2ZM/A/thunderbolt-3-usb-c-to-thunderbolt-2-adapter.
The second one is needed for the more recent Macs.There is not an adapter for FireWire straight to USB-C.
Now assuming that you are using iMovie 10.
If you are unable to capture DV tape into iMovie then you could try to capture using QuickTime .This has worked for some on here where iMovie has failed. One long clip will be generated rather than the individual scene breaks that iMovie would detect. Look at the record feature in this.
https://support.apple.com/en-gb/HT201066
Now for the Hi8 tapes.
If your DV camcorder does have the pass through feature, then you would need to set this feature up on the camcorder menu.
As you would be using the DV output port on the camcorder then you would need the same cables as above.
You would need to try importing using QuickTime Player, as mentioned before, as it is very unlikely that you could use iMovie for pass through.This is because time code may not be present from the pass through which iMovie 10 requires.
You could try iMovie first since your cam' just may provide time code for pass through.
As you have many Hi8 and DV tapes then I can see the attraction in getting these onto your Mac, and the dilemma is of course whether you can achieve this after spending may be up to $80/£80 on adapters.
Let us know what year Mac you have, model and the OS, as the more info the better and also if you have an older Mac with say iMovie 9 or even 6. If a friend has an older Mac with older iMovie that you could borrow then you could use their Mac with an external drive to capture. You return their Mac and you keep the external drive to work with.You would probably need an external drive anyway to capture as you have many tapes......13 GB per hour of DV tape.
Access to an older Mac/iMovie with a FireWire port would be ideal.
If I were in your shoes I would visit an Apple Store ( hopefully you have one convenient to you) and have a chat with one of their advisors.Explain what you want to do and ask if the adapters could be returned to the store in "as new" condition if you are not successful with the capture.
The output quality can be quite good, better if the cam' recorded in SP rather than long play.
Also better if the camcorders you use for tape capture into iMovie are the same camcorders that did the recording.
The DV will be better of course than the Hi8 but we have all now got used to HD e.t.c so looking back at standard def may take some getting used to. But it is what it is.
Let us know what kit you have or what you may be able to borrow( OS and iMovie version ) and also model of the DV camcorder.
