Mine are all insured separately. Multi car policys never seem to cover exactly what you want, avoid trade unless you have a business, as the value will be low on the car if you need to claim, then they might want vat receipts.
Most good insurers will mirror ncb onto other cars. Both the megs are insured via greenlight, gt turbo is on a classic policy, clio’s are both insured on one policy with elephant. All in all the lot is under £1k.
I wouldnt bother with laid up insurance unless its worth a lot of money, and then might be best off just having a policy running. My mate has a 3 door cosworth, and he used to have the laid up insurance for winter, but now he just has the policy running all year round.