Only very generally:
get trees that respond well to being espaliered
there will be a slight difference as no wall to give wind shelter or warmth
Self-pollinating or you can pair trees that can pollinate each other
If you want to mix nice climbing plants that flower at different times to the fruits, give the fruit trees a year or two head start before adding the not-too-rampant climbers.

Multi species graft trees need to be very well balanced, and a lot aren’t. You also need to be aware if pollination partners if there aren’t a lot of other fruit trees about. The main thing for trained trees is where they fruit - training involves shortening branches, so if the fruit forms at the tips of the branches you won’t ever get any because you’re always pruning though off.

thank you. Well the pergola is with a mind to aesthetics but in a small garden then usefulness is important: so yes food. I have the design skill of a long dead cow so it’s never going to look show standard. But I’m not too bothered by that

I've had repeated dissapointing results from multi-grafted fruit trees.
You may have better luck.

I'd personally suggest trees grown on a decent dwarfing root stock from a reputable supplier (my allotment 'orchard' came from orangepippin.com if you're in the UK)

I’ve got 3 vines in for now but I did wonder about another one maybe. Monty said this year to remove 2/3rds of the immature fruit to get a decent crop so I will be trying that…

that’s fantastic!!! It must look extraordinary???

I was thinking about Kiwi or Kiwi berry (the latter ones are more suitable for colder environments afaik, I once saw someone growing a stunning amount of them in northern germany)
They could really like the microclimate of the pergola I think.
Edit to say: cool Project btw! I would love to follow it along under the #Gardening

Like another comment I wondered about grapes. But possibly the severe pruning would get tricky if the goal is to cover the pergola?

I've seen mixed reviews on multi-graft fruit trees, with stories of poor survival of some grafts long term and low productivity. That being said, I have personally grafted multiple varieties on established apple trees and they are seem good. I do have many pollinators trees and have only worked with apples (imo the most forgiving fruit tree type).

This might be too much of a rabbit hole, but when I am looking for a useful or edible plant for a particular spot, I always check Plants For A Future. The website lets you filter by a bunch of criteria (edibility, hardiness, habit, etc). Sometimes seeing a big list prompts a new idea.

https://pfaf.org/user/

Deeply consider overall height both of the pergola and of the trees. I see so many archways etc, where people didn't consider space for the plants and clearance for walking at full height. You need at least 2 ft of clearance above a person's head. Aim for semi-dwarf and dwarf rootstock on any fruit trees you might plant. Hope that helps. Specific trees would really depend on your solar aspect.

I think multi variety trees are great in small gardens, I had a three variety bush pear, that gave me early, mid season and late pears. But not sure it would work in Espalier mode.
Suggest dwarfing root stock and plant maidens, one year old will get away quickly and you put the layers where you want them.
As others have said suggest you go for cross pollinating varieties, and find a grower who does their own grafting and has the stock to suit your area.

I have to ship everything in - we have no growers on the island (#Jersey) any more as far as I know. Makes it a bit tricky.

ah a warmer( ? Drier ) climate than mine (South Wales)! Some French growers are probably your best bet, Brittany has lots of fruit growing as does Normandy, though not sure if they cater for the amateur grower rather than the commercial ones. But might be worth asking, many of the great varieties grown in SE GB started as French or Belgian varieties.

yeah - it’s a good shout. We get over there regularly and there is a massive nursery with a lot of trees outside St Malo. That is a fair suggestion - I’ll see if I can get over.