I just googled the con color fir. This tree easily gets 50' or 100' tall with a spread of 20'. How big are they now? Anything you plant now will be covered by the firs in a matter of time.

They are about 5 feet tall, and I spaced them for growth. Was just hoping I could fill in while they are growing. Groundcover would be great if bushes don't work. I'm a novice, so any help is appreciated.

I think your trees are Abies concolor, trees that are grown here in CT as a newer variety of Christmas tree. These have a very distinctive citrus-like scent and longish, soft blue-green needles. This variety grows slowly so you don't have to worry about tremendous height for many, many years.

I'm also not clear about what you are want as companions "plants that are less maintenance". Are you thinking of shrubs, perennials, or ground covers? Could you post a photo of the trees? It's difficult to make suggestions without seeing them in their environment.

Molie

People sometimes plant inexpensive, fast-growing deciduous shrubs as fillers while waiting for evergreens to grow in. Things like forsythia should be really inexpensive now, since they're done flowering (especially ones that were brought into flower early for spring sale), and they grow really quickly with a little water and mulch. They're more or less no-care, other than watering for the first season, and they can be removed when the conifers fill in around them. The only caveat is that you have to keep an eye on them as the firs mature, to be sure they don't interfere with the evergreens' growth.

If you stagger the planting, with the deciuous shrubs in front and between the firs, you can use a more choice plant and leave it in place - this can still give you a visual barrier quickly, if that's what you want. Something like clethra might be a good choice - it's a native, and tolerates shade and droughty soil. Check the CT Botanical Society page on shrubs, or a reputable local nursery (I don't mean Home Depot!)

One other piece of advice... if this is an area that used to be lawn, don't just dig holes for the shrubs/trees, rototill the whole area and mulch it. You can often hire someone to do this for very little money. Everything will grow better and faster, and the tilling will pay for itself many times over.

Here is a link that might be useful: CT Botanical Society - shrubs