I think palms look best not planted next to each other, unless they are palms of the same species, or palms that look nothing alike and don't have the same growth habits. So I would break up the plantings with other nice subtropical looking plants in between. In a zone 6b you can get away with some nice tropical looking perennials that will overwinter in the ground like kopper king hibiscus, different types of lilies, and things like that. You can mix those with things that you can dig up in the fall like ginger, Colocasias, Alocasias, and cannas. Castor beans are awesome if you want a big plant for a ridiculously cheap price since you can have an 8 foot tall plant from seed in just one growing season. They come in green and purple and the only catch is they are extremely poisonous so make sure you don't have any curious pets or kids who can get to them.

Trachys look great in groups of 3 so if you have the room I would have 3 different sized ones growing relatively close to each other. If I had the room for that arrangement I would definitely do that! Just make sure to give a good 5 feet of room between them and that they are varying heights so they don't get a cluttered look!

Good luck! Sounds like you have a nice collection going!
-Alex

Awesome ideas! Thanks!!

Good ideas from Alex, but I will further warn against castor beans. I decided to quit growing after 1 season, because they get too big and aggressive. A good point is that you can prune the bottoms to allow other stuff to grow underneath, but when you dig them up their roots are very aggressive, and there is a lot of material that you will need to dispose of. I don't know if you technically can't, but I assume it's unsafe and I definitely wouldn't compost it or burn it because of it's highly toxic qualities. Unless you have a big yard with lots of space to fill and someplace to dump them, buyer beware.

If you're looking for something smaller, and cheap because they are easy to overwinter in a potted plant in the house, try purple heart or wandering jew, and Persian shield. All are easy to grow from cuttings rooted in water. Mexican heather (ruellia) is also easy to overwinter and almost weedy, although not hardy here. Dahlias are also tropical looking, but I've never been able to overwinter (they supposedly are easy to dig and store.)

Also on the smaller side, I am a fan of big season-long color (oranges and purples are my faves), fragrance and variegated foliage, so some other smaller options that are hardy or annuals might be cupheas (I personally like Mexican Cigar Plant (Cuphea Ignea Coan Scarlet the best, also Mexican heather, both are widely available), eucomis, coleus, zinnias, African daisies, and heliotropes. Roses always work too.

For larger plants, don't forget brugmansias and plumerias (they can get very top heavy), and also don't discount tropical hibiscus. The tropical hibiscus have very shallow roots and take to aggressive pruning making them easy to dig. They do tend to get some sort of nasty funk on them by Jan/Feb, but a good hose down and spraying with neem oil or an organic fruit tree spray takes care of them.

Good ideas also brad! I just wanted to agree on trying out Brugmansias, plumerias, and tropical hibiscus. They are all great plants! Purple hearts are in my yard every year and add a lot to a groundcover. I only use them in parts of the yard that are very neat and clean though because they can look messy (it's a great look when mixed with neat and big leafed plants but it might not look as nice among small leafed plants). Purple hearts have come back for me from the ground for the past 4 years now. Some are already popping back up inside the palm tree enclosures despite the rough weather outside.

As far as Castor beans go, I would never burn them, but composting them should be okay. They are only toxic to animals and humans, they aren't damaging to other plants so they will be fine for composting. From what I have read you can even plant edible plants near castor beans (as long as the edible plants don't touch the actual plants). I personally wouldn't do that though!

Good luck!
-Alex