Yes “gearing” refers to the sizes of all the cogs on the cassette. You really only care about the largest cog. If the large cog on the trainer is bigger than the large cog on the wheel, the chain may be too short. That may prevent you from getting into the lowest gear, or you may get into that gear and not be able to shift out of it, or it may rip the rear derailleur off the bike when you shift into it.
Brilliant.
So… after the great advice decided to get the Carrera Karkinos (for missus outdoors & myself for indoors), then the exact same cassette (Shimano CS-HG200 8-speed, 12-32T), which I will then be connecting up to a Wahoo Kickr Core.
Question: Will I need a front wheel riser block?
Will be removing the back wheel for my use (quick release), then disconnecting from the Kickr and attaching back wheel when missus goes out (which won’t be every week so its going to be now and again).
I think that ends up being the best way to 1) Buy missus a mountain bike and 2) Get me up and running as simply as possible with a great home trainer … but happy for someone to chime in and correct me if anything above is wrong.
With DirectDrive (DD) trainers, riser blocks are not needed. They usually come, or at least my Saris H3 came, with a very low profile piece of plastic for the front tire to rest. I suppose the purpose is to minimize scuffing of the tire. If you use a thin pad, the plastic does lock down the front wheel and keeps it stabilized from turning every which way. If you find yourself too low profile for comfort a riser for wheel on trainers are sold separately which will add ~3" to the front wheel height.
Thanks. The wheels are 27.5", will just place the bike wheel and trainer on a mat (and then some plastic if needed).
I’m back.
I now have a Kickr Core. Waiting for the Shimano CS-HG200 cassette that comes on Saturday. This matches the exact same cassette on the new Carrera Karkinos bike (and as it happens also my son’s Carrera bike too - lucky eh).
So, simple question I think. Do I need to use a spacer with the Shimano CS-HG200 8 speed cassette? And does the one that comes with the Wahoo Kickr suit that?
One 1.85mm spacer (proper Shimano one is best, the part number is FH-9000 or Y4T724000), situated behind the cassette.
Like this one? (reason I’m checking is it says for 10 speed cassette, but mine is an 8 speed cassette).
Really appreciate the help, its all new to me as a non-cyclist 
Yep, but just buy one here: Shimano FH-9000 low spacer, 1.85 mm 4524667943969 | eBay
Purchased. Thanks for the help.
Well, I think I’m now just waiting for the cassette and spacer to arrive. Then can get my kid to put his bike on the trainer and test things out
Then, on Thursday I pick up the adult mountain bike and let the real fun begin!