I'd consider McKean's hebe (Hebe mckeanii). It has a very bright, grassy/limey green color and forms a tight, rounded mound about 2'x2'. I think the coloring is a little more suited than a more yellow tone, as well. Would make a very nice textural contrast to the weigela, although I'm not at all sure about the advisibility of alternating the two. It's not a look I particularly advocate, but that's just my very personal opinion :-)

I have Spirea 'Magic Carpet' and 'Lime Mound' they both get far bigger than the Weigela. 'Magic Carpet' is bigger than 'Lime Mound' a lot bigger than I wanted them to be. Hard pruning in early spring seemed to encourage them to grow even bigger.

There is the new 'My Monet' weigela but they are expensive....

You can't get a straight answer on the euonymus because it does whatever the heck it wants to as the mood strikes it. Basically it's a woody creeping mounding vining thing. I've seen it mounded up and sheared into gumdrop shapes, and I've seen it as an undulating groundcover, and I've seen it climbing up fences and tree trunks. It's a shapeshifter.

I'm not big on the alternating soldiers look either, tending more towards the groupings of complementary plants with some accents. I have a wine n roses weigelia planted with goldmound spireas and I like that look.

There are a lot of yellow-foliaged dwarf conifers available, it might be nice to have something evergreen in this border? I also like my variegated yellow and green holly in the shade border with rhodies and camellias. I think the Hedgehog holly has yellow variegation and stays small, but very spiny! Depends on how much sun this border gets. Lots of hot sun won't be good for things with yellow foliage as they're more susceptible to sunburn.

Some yellow shrubs sunburn and some don't. Many kinds to choose from these days.