There are many species suitable for bonsai. I am going to concentrate on varieties of Ilex vomitoria for they are what we use the most. All growing conditions mentioned here will be suitable for all Ilex’s.
Ilex vomitoria Yaupon Holly This Holly has dense compact growth and small scalloped leaves. The female produces small clusters of red or yellow berries.
Ilex vomitoria ‘Schillings Dwarf’ Tight compact growth with leaves about two inches. Flowers in spring but generally doesn’t produce berries.
Ilex vomitoria ‘Nana’ Dwarf Yaupon Holly Almost identical to the Schillings Dwarf, their leaves are slightly smaller and it does produce berries. The easiest way to tell the difference between Nana and Schillings is the new growth on the Nana is green as where the new growth on the Schillings is red. Berries only appear on female plants.
Ilex vomitoria ‘Pendula’ Weeping Yaupon A dwarf weeping Holly that does berry.
PRUNING... The top can be heavily pruned as long as you leave a few leaves on each stem. More so on collected material as to nursery grown material. I will prune the top back and then wait until it starts flushing out to prune the roots. On large cuts, spray with Lysol and then seal. If removing a large branch on the top of the tree, do not use your concave cutter. Leave some stub and then carve that back so it is somewhat rounded up to repel water. Water can sometimes enter a large cut and rot out the center of the tree.
TRAINING... Wiring can be done on secondary and tertiary branches. Branches tend to get very brittle as they age. Clip and grow works well. Prune back to two leaves for tight growth. They grow quick so will require periodic pruning through the growing season. Be careful not to allow the branches to get too full of foliage for that will block the light from the inside of the branches and decrease air circulation which will result in your inner leaves dying off.
WATERING... Do not allow to dry out. I water mine well through the growing season and less through the winter.
LIGHT... Full sun, starting with the morning, for if their leaves stay too wet through the morning they can suffer a fungal die back of their inner leaves. They can suffer some heat stress in July and August so I give them a break from the afternoon sun.
INSECT/DISEASE... Leaf miners can be a problem and if stressed scale and mites can occur. Sometimes suffer a systemic fungus problem that can kill a whole branch and sometimes the whole tree. At second repotting wash all of original soil off, that which came when collected or from nursery pot.
FERTILIZE... A heavy feeder through the growing season. I use Osmocote and feed bimonthly through the summer with a water soluble 20-20-20 fertilizer.
REPOT... Can be repotted from spring through summer. On first planting be careful not to take too many roots at once, generally best to reduce root ball over several plantings, removing 1/4-1/3 at a time.
SEASONAL... Cold hardy here.