"6 to 8 feet" isn't really a tree.

Here's a few medium to large shrubs that you can "limb up" to create a multi-trunked tree look; although they'll want to get taller than 8 feet, you can keep them at 8 with creative pruning:

Acca (Feijoa) sellowiana PINEAPPLE GUAVA
Arbutus unedo 'Compacta' STRAWBERRY TREE
Arctostaphylos ÂAustin Griffiths AUSTIN GRIFFITHS MANZANITA
Arctostaphylos densiflora ÂHarmony HARMONY MANZANITA
Arctostaphylos manzanita ÂDr. Hurd DR. HURD MANZANITA
Arctostaphylos pajaroensis ÂParadise PAJARO MANZANITA
Banksia ashbyi ASHBYÂS BANKSIA
Banksia brownii FEATHER-LEAD BANKSIA
Banksia coccinea SCARLET BANKSIA
Banksia menziesii RASPBERRY FROST
Callistemon citrinus ÂMauve Mist M. M. BOTTLEBRUSH
Callistemon citrinus LEMON BOTTLEBRUSH
Callistemon phoeniceus LESSER BOTTLEBRUSH
Callistemon rigidus STIFF BOTTLEBRUSH
Callistemon salignus WHITE BOTTLEBRUSH
Cercis occidentalis WESTERN REDBUD
Comarostaphylis diversifolia SUMMER HOLLY
Cotinus coggygria SMOKETREE
Escallonia laevis PINK ESCALLONIA
Escallonia x exoniensis ÂBalfouriÂ, ÂFrades HYBRID ESCALLONIA
Grevillea asplenifolia FERN-LEAF GREVILLEA
Grevillea ÂHoneygemÂ, ÂIvanhoeÂ, ÂLong JohnÂ,
ÂPoorinda BlondieÂ, ÂRed HooksÂ,
HYBRID GREVILLEA
Grevillea hookeriana TOOTHBRUSH GREVILLEA
Grevillea olivacea OLIVE-LEAF GREVILLEA
Grevillea victoriae ÂMurray Valley Queen M. V. Q. GREVILLEA
Hakea arachaeoides SPIDER HAKEA
Hakea bucculenta HOT POKERS
Hakea francisiana PINK SPIKE HAKEA
Hakea laurina SEA URCHIN, PINCUSHION TREE
Leptospermum lanigerum WOOLY TEA TREE
Leptospermum polygalifolium ÂYarra River RED SPIRE TEA TREE
Leptospermum scoparium ÂHelene StrybingÂ,
ÂKeatleyiÂ, ÂPink DamaskÂ, ÂPink PearlÂ, NEW ZEALAND TEA TREE
Leucospermum reflexum RED ROCKET
Melaleuca armillaris FEATHERY MELALEUCA
Melaleuca bracteata WHITE CLOUD TREE
Melaleuca decussata LILAC MELALEUCA
Melaleuca elliptica GRANITE BOTTLEBRUSH
Melaleuca ericifolia HEATH BOTTLEBRUSH
Melaleuca hypericifolia HILLOCK BUSH
Melaleuca nesophila PINK MELALEUCA
Pinus contorta contorta SHORE PINE
Pittosporum crassifolium KARO
Pittosporum eugenioides TARATA
Pittosporum tenuifolium KOHUHU, TAWHIWI
Prunus illicifolia HOLLYLEAF CHERRY
Psidium cattleianum STRAWBERRY GUAVA
Regelia velutina BARRENS REGELIA
Rhaphiolepis ÂMajestic Beauty (multi) M. B. INDIAN HAWTHORNE
Viburnum suspensum SANDANKWA
Xylosma congestum SHINY XYLOSMA

Joe

This website has most of the available crepe myrtle varieties listed, along with their respective heights.

Here is a link that might be useful: crepe myrtle varieties

Zuni is a small Crape Myrtle.

Outside of the Crape Myrtles, as Joe said, 6-8' will be hard to find in a tree. Your best option would be to find a shrub and trim it like a multi-trunk tree.
You could also use something like a dwarf citrus tree.
Loropetalums look nice trained as a multi-trunk "tree" form.

wanda

Thanks so much for your help! That's an amazing list, gardenguru, and the crape myrtle chart is fabulous. I definitely see a limbed-up shrub in our future!