I'm working on a tiny hallway addition and I'm using ICFs for the first time. I'm curious what the best practice is for "connecting" and ICF wall to an existing foundation. Should I be bringing the ICFs up to the exact same height as the existing foundation or should I finish the last foot or so with traditional cement blocks? Do I need a bead of foam in between the ICF and the vertical foundation or will the concrete seal on its own?

Here's a photo of the job in-progress.

Not an expert but you can clean the wall and spray foam the block to the wall and later you will have to seal it with butyl tape or a liquid sealer.

I think you need anchor bolts from the inside going into the new wall.
https://www.homedepot.com/p/Mutual-Industries-1-2-in-x-18-in-Galvanized-Anchor-Bolts-50-Pack-7701-0-18/304132473

You will have to do the whole wall and the joint with water sealing. It will NOT seal on it's own.

You will need to hold the new wall to the old wall with strapping and/or back fill, might be able to wire the wall to the anchor bolts with rebar wire.

I think you are supposed to have at least one layer of rebar at the top with that height, maybe 2.

I would go even with the top of the other wall...you can run the block through a table saw twice use each set of interlocking teeth. Looks like this will get to your height and minimize your waste.

duplicate....sorry

Common practice (at least in our area) is to dowel & epoxy rebar into the existing foundation to tie the new & old foundation together structurally. WP membrane should extend past the point where the 2 foundation meet onto the old foundation at least a few feet (a primer may be needed on the existing wall for proper adhesion). If water issues are a concern you can also install a bentonite strip at the cold joint (this product expands if it encounters water so any moisture that migrates past the membrane will be stopped at the cold joint by the bentonite).

Height is typically determined by finished floor height of new vs existing area. IE, if floor elevation is supposed to match, just be sure ICF ends with enough space for depth of new floor joists, sill plate, floor sheathing, etc.