Hi Hetty! Very nice big pot! I also use big pots for my little garden..my initial problem was what to put below, since the Cacti and Succulents roots do not run that deep anyways..and I read somewhere to use empty bottles or inverted old pots underneath ...I did that and at least saved me from using way too much soil.
Other consideration though, how to keep the balance, I do not know if it is the soil sinking in between or it was the wind pushing them down but my cacti is like a leaning tower right now just trying not to distrub it this winter, will try to tackle it in spring
This is how my leaning cacti looks right now:
Would be nice if you can get tall big faced succulents, the smaller ones seem to appear smaller inside the big pots..well, that's just my personal observation on my own plants.
This is a photo of another aeonium I have, this one stays dark always, and a bigger succulent in front of it. The other pot to its right just have jade plants.
Putting cacti or succulents that grow taller adds good dimension..or maybe add ones that trail down around it like sedum burritos for accent?
)
It took two men to lift that thing where it is now 
