I've changed my preferences on fir - Jorg Kohout suggests:
Abies koreana
Abies veitchii
Americans typically use Nordmann/Balsam/Canaan

2-needled pines:
sylvestris seems to be universal.
what else are you using? especially for dwarfs/mini's?

5-needled Pines
Again, Jorg Kohout suggests:
Pinus armandii
Pinus wallichiana
Americans use exclusively strobus.

3-needled and combinations of 2-3-needled pines:
A friend in the US has found Pinus ponderosa to be a very useful understock.
Pinus strobus also seems to be a US choice.

Spruce:
Norway is much easier to work with than the sharp needles of pungens.

Larch:
What are you using? decidua I assume?

Cedrus (true)
Cedrus deodara has been reported by Bob Fincham as a zone 5 "root-hardy" understock.

Cypress:
Here in the US often cutting-grown rootstocks of Juniperus 'Hetzi' are used. They can be propagated early winter and grafted onto late the same year.
Any other Cupressus info?
Microbiota I've heard is very compatible with (at least) nootka. That's a 'wild' combination however. Nootka grafted to microbiota shows excellent compatibility after 20 years.

The concept of 2-3 needles hard pines (rigida, ponderosa, bungeana) being grafted to another pine of the same needle count, makes sense to my friend. It also makes sense to me.

Any more thoughts? It would be much appreciated. I'd like to learn a lot. (other genus and/or species combinations)

Thanks,

Dax

"3-needled and combinations of 2-3-needled pines: A friend in the US has found Pinus ponderosa to be a very useful understock. Pinus strobus also seems to be a US choice" - yes to Pinus ponderosa; but not Pinus strobus, except for Pinus bungeana and pinyon pines (these are more closely related to white pines than to P. ponderosa, etc.).

"Nootka grafted to Microbiota" - that's just weird, grafting a common plant (Nootka Cypress) on a rare rootstock (Microbiota) . . . I could understand the other way round!

Resin

Dax

For firs, I use abies alba and abies koreana.
For 5-needled Pines , I use pinus strobus,pinus cembra and pinus peuce.
For pinus cembra, I graft them on pinus sylvestris understocks with good results.
For pinus parviflora Glauca, I graft them on pinus contorta var. latifolia understocks.
You can graft pinus koreansis and pinus pumila by using pinus sylvestris understocks.
For 2-3-needled pines, I use pinus sylvestris understocks.
For pinus ponderosa, I was told that it is better to use pinus ponderosa and pinus nigra understocks
For pinus virginiana ,I was told that it is better to use Pinus banksiana understocks.
For Spruce, Picea abies understocks.