I've found that established plants are a lot more forgiving in terms of soil than seedlings. One year I planted a bunch of seeds in the very coarse 'soil improver' compost I got cheap from the council recycling plant, and they did very badly indeed. (In fact that compost is pretty rubbish all round; some plants tolerate it but it does better mixed in with other things.)
Having said that, I've found that any fairly fine compost/soil that hasn't dried out (if you leave pots unwatered for a long time the soil can go very dry and dusty) will do the job for seedlings. Do you make your own compost? You can sieve that out and use the fine crumbly stuff that comes through the sieve for seedlings, although beware of rogue seeds from other plants that went into the compost pile.
If you need to buy soil/compost, in theory both seedlings and bigger container plants will do better in dedicated potting compost. If you're buying 'general mix' (non-peat!) potting compost, you can mix it up with other things to get a better potting compost; this is the one I suggest in the book:
• 2 parts mature compost
• 1 part vermiculite (to increase porosity).
• 1 part coarse sand (aka builders’ sand) (to ensure good drainage).
• 1 part coconut coir (to boost water retention).
More often I've mixed general mix compost in with old potting compost, my own worm compost, and a bit of sand when available. I've never had the option of mixing in garden topsoil but my mum who has a nice big garden starts seeds and plants them on in a mixture of garden soil and compost and gets on well with that. The more established the plant I think the lazier you can get away with being.
I would certainly use that mix for both starting seeds and growing them, but I think for starting seeds especially this is something that gardeners will happily disagree on... If you've had good luck with a specific mix for ornamental seedlings then give that a go.
That's for veggie planting in pots; I don't however know if ornamentals would need something different as I know very little about non-edible plants I'm afraid
My guess is that the potting mix above would be fine but I don't know if those have different requirements. I'd guess that you might have to wait until next spring before you plant them for them to stand up to being transplanted by the owners, but again I'm not entirely sure. Personally if I was taking a cutting of an established plant I would tend to keep it in its pot for at least a few months and possibly pot it on once before putting it in the ground.