Hi, Michelle. I cut my Limelight back by 1/3 and fertilize with 20-20-20 in the spring, but I didn't cut it back at all the first spring. That's for established plants, IMHO.
I have chocolate Joe Pye weed next to my Limelight and I think that's an interesting combo.
I have a picture and am working on figuring out how to post. Karen the technodummy

In my opinion, deep vivid colors (especially deep purple) look best with Limelight. A very dark green foliage backdrop such as a conifer also looks great. Those deep contrasting colors really makes the lime green pop.....yg

I'm not an expert on companion plants. I tend to focus on having the right plants in the right place. I do try to have seasonal blooming times vary in an area though. My maintenance schedule is to prune off any winter dieback in Spring and fertilize(Miracid is best for my area)

Here is a link that might be useful: limelight

Since my limelight gives some shade underneath I place hosta's with great variegation and yellow in color.
Hosta: Stainglass, Daybrake, June, Pauls Glory, Reveolation, Patriot etc.
Because these hosta's can take sunshine and dapple light I think they are a nice combo to bring out the color of limelight.
Using the hosta's will give you great color combo before limelight begins to bloom.

What about underplanting Limelight with a row of one of the following:

- Heuchera, 'Palace Purple'
- Pennisetum 'Karley Rose'
- Hakonechloa 'Aureola'
- Fescue 'Elijah Blue'

?

I have Shiso perillo planted around two of mine - I like the contrast - Carrie

Agree with the above....Heucheras and hostas depending on sunlight. Also, something for texture as I think lilac leaves are similar in size to hydrangeas. That way you have interest even when they are not blooming. Since you have them planted with Lilacs I'm assuming they are getting full sun? I also like going for different shades of green.....lambs ears, fescue, blue spruce...Dark greens too.

A big swathe of Lady Fern around hydrangeas are beautiful.