Im in the EU. so i might be wise to wait for some-one with US espalier experiance befor following my advise.
I dunno how much it would differ. But i somtimes see quite the difference between advise and data between the US and EU fruit growing.
Rootstock and interstem
With your souil MM111 sounds like a good choice. You could interstem it with a 4-12 inch piece of B9, M9 or even M27. This would dwarf it, longer interstem=more dwarf. Depending on interstem length you should be able to get roughly M26 size. But still have the MM111 roots. And this would be a way more manageble size for espaliers.
Scion vigor
Most sources i will link go to a single (the same) apple variety. But the websites lists way more varieties. Use the search fungction on each website.
Is an american source. I think i would trust this most for your situation.
looking under “fruit ID” just under the name of the variety. And scrolling down to “growing”
suplementary table 2. Will download sheet. You can find the ploidy here of 600 apple varieties. 3n = triploid, almost always a stronger grower. 2n is diploid.
Triploids canot fertilize themselfs, or other apples. And thus need a diploid apple nearby with the same flowering time.
Usualy also lists vigor of the tree.
use google translate for these.
https://library.wur.nl/speccol/fruitvrij/aepfel/Aepf1/Ae027.htm
https://library.wur.nl/speccol/fruithof/fruit/App/Tekst/AppT03.htm
different sources different vigors
As you might notice, different sources list different vigors. It is hard to judge which source matches your situation most. But in the end it doesen’t matter that much. As long as your not putting a really low vigor variety at the bottom and a triploid super vigerous in the top. you can balance them by pruning. It is however nice, where you can find it. To place them in order of decreasing vigor bottom to top of the espalier. (starting botom most vigerous, ending top least vigerous)
Btw i just picked ananas reinette as an example because i mentioned it earlier as an example. Does not mean that would be a perfect or even good apple for your situation. I dunno how it would work for you.
Links, links and some more links
most of these links i also mentioned in this topic.
It also contains a lot of links of what i consider to be good sources to use when learning to graft. Might be worth checking them out. Their in the “links” section of the topic.