Cotneaster is good, perwinkle can and will overtake the cotonester years down the road unless you keep it in check, sedum is okay, I prefer ajuga. Why don't you look at eunoymous fortunei, Emerald Gaiety, it will be evergreen, and add some height, through the winter besides having mutlicolored leaves. I'd also look at juniper I like Blue Pacific. If the soil is good the plants will eventually run into each other so you will be pruning in the fall 5 years from now. You are in MA so you will not have the growth I get in the Carolinas but I must prune twice a year.

should I mix it up

Up to you, but I always mix it up. Sometimes I plant multiple GC's to see which one "wins" and covers the best. In many cases, I find that multiple species can co-exist, and create some interesting effects.

Examples of some good combos I have used successfully:

Delosperma cooperi with various Sedum.
Fragaria chiloensis with Potentilla neumanniana.
Small leaved Hedera with Ophiopogon.
I'm about to try Euonymus fortunei 'Colorata' with Campanula takesimana.

Mix it up; you might like the results! Cheers!

And for yet another opinion.......I don't recommend it :-)) By definition, a good groundcover is an aggressive spreader and you run the risk of one, more aggressive type overwhelming another, less aggressive species if they are interspersed or planted very close. What I can recommend is relegating different groundcovers to separate areas, especially if a large planting area contains a variety of shrubs and other plants. Plant individual types in large swaths or patches. Eventually they will merge, but you are better able to keep on top of rampant growth if you give each their own space and some distance beween.

Of course, Gardengal is right that groundcovers can be aggressive spreaders, and can overwhelm the competition. But that ability to overwhelm is not absolute. I refer back to my original post; I purposely plant multiple groundcovers in an area specifically to see which one will overwhelm the other(s); which one will "win", as I put it. The philosophy here is that the most adaptable, fastest growing, tallest, etc. plant will overwhelm the rest.

In the real world, however, I find that this is often (never?) not the case. The examples I list above are groundcover combinations that currently happily co-exist in my garden. The effects can be very pleasing and interesting, and have some practical implications as well.

Let me further expound on one such combination that I am using in a large raised bed. I planted Fragaria chiloensis with Potentilla neumanniana in close proximity. Both are prostrate groundcovers with similar (but not identical) cultural requirements, and roughly the same height. Once established, the Potentilla is somewhat more vigorous and dense, but the Fragaria is slightly taller, popping up in random patches and drifts through the Potentilla. The visual impact is quite nice. The Potenilla leaves are small, lacy, and a light green color. The Fragaria leaves are larger, rounder, and darker green. The contrast is outstanding.

Now, there are multiple practical advantages to this combination as well, beyond the aesthetic.

1) Both are drought tolerant, but the Fragaria is somewhat more so. The Fragaria tends to colonize the drier areas of the bed, while the Potentilla dominates in the wetter ones. They happily coexist in the area that has optimum moisture for both. With the combination, I can completely cover areas that have different moisture profiles; without the combination, I may have poorly covered, or even bare spots.

2) They flower at different times. The Fragaria has a flush of white spring flowers, and then the flowering diminishes (for the most part). The Potentilla flowers all summer with its small yellow flowers. Each will have sporadic flowering year-round. I have good flowers spring through fall; with each dominating a different season than the other.

3) The Fragaria is more cold tolerant that the Potentilla. In winter, the Potentilla begins to brown somewhat, but the Fragaria stays dark green. I still have some winter interest with the combination. The Potentilla rules the summer, the Fragaria rules the winter.

4) Each has slightly different shade tolerance. The Fragaria will grow in shadier areas than the Potentilla. The Potentilla is the more aggressive species overall, but the Fragaria is more suitable for shady spots. Again, this results in fewer bare spots than if I had used only one species.

To add to the mix, I have some prostrate Lantana and Sedum interplanted in the mix. Both of these species can be used as groundcovers in their own right, but only if used alone or with other compatible plants. But, the Fragaria and Potentilla completely overwhelm them. However, the Lantana dutifully pops up through the groundcover each spring, resulting in random patches of nice foliage and purple flowers above the other plants (though not vigorous enough to choke them out). With the groundcover competition, this Sedum grows unusually for its type; it grows taller and flowers more profusely than a monoculture of this plant. It is incapable of overwhelming the other groundcovers, but adds some scattered interest throughout the planting.

I'm not trying to discount what Gardengal says. I'm just saying that in the real world, groundcovers can and do co-exist, with some interesting aesthetic results and practical advantages. Certainly, if you have a formal garden, a monoculture might be recommended (for visual reasons). On the other hand, if you're like me, you like the more informal, forest-like quality of multiple plants growing in the same space.

Cheers!

. . . I forgot to mention that I also have a prostrate Rosmarianus growing at the edge of the planting, in the driest part of the bed. It happily coexists with the Fragaria in the drier areas, but tends not to want to colonize the moister parts of the garden.

If you add it all up, I have a total of 5 species of plants that are considered to be groundcovers growing in the same area, with different results depending upon location in the plot.

Redneck, you may have hit on the one combo of groundcovers that will coexist very well together! Fragaria and potentilla are extremely compatable - so much so that they have been interbred, with the result being plants like Fragaria 'Lipstick' or 'Pink Panda', integeneric crosses between the two plants that offer the best charcteristics of both.

And I will also say that combining multiple groundcovers successfully does depend on the space you have to work with. With few exceptions, I'd hesitate to interplant two different species, but if they can each be planted in individual swathes or drifts with some distance in between them, it can work very well.