Most mail order minis come in small pots and will grow up to 2 feet in time. When you receive them planting them immediately into a very large pot may spell trouble from overwatering. It's usually better to increase pot size as the minis grow to make sure you don't overwater their small root system. Your climate is a lot different then mine so I can't tell you when it would be best for you to transplant them into a large container and surround them with filler. A lot of it depends again on the size of the mini you receive initially and how fast it fills out.