28mm clinchers will be fine on your wheels. My Giant came with PR-0 (iirc) wheels, which are tubeless ready but they were set up with clinchers and tubes when the bike first arrived. I've since converted to tubeless.
If the bike is coming already set up tubeless, why not give it a go first before changing, see how you get on with it?
Yes bike is set up tubeless but as stated above I've no idea what to do with them if a deflation happens? Do you put sealant in or does it come already enclosed in the tyre?
If the sealant works it should stop full deflation on anything that doesn't need plugged.
If you get a partial deflation that the sealant stops but you'd rather not ride on then you just need to top the air up with a pump.
If you don't have plugs and get that level of deflation then it's a case of: tyre off, boot over the hole and put a tube in to finish the ride.
Check with the shop when you collect it to see if they've put sealant in or just pumped the tyres up, I'd expect the former but...
For plugs, there's various options, I went for one of these
https://www.3peakscycles.com/products/p-repair-dyna-carb-ultraand some extra insertion tubes as the loose plugs stick together in the barrel...
Not had to use it yet, and never had a need for similar when MTBing (or had to put a tube in either).