Interesting to note that nursery owners in South Africa are little devils
Got these from a fiends nursery today.

I sometimes think it is the same over here
They look like great trees to start working with. Olives are very tough so make great, hardy bonsai. I do get a bit tired of cutting off all the suckers though.
I hope you are planning to style the acacia as a 'savannah' type tree - tall, spreading with a flat top? Or do they naturally grow in other shapes if there are few giraffes?
The potting mix looks a bit sad compared to what we use here but I know you can get used to keeping plants alive in almost anything except concrete. Do you have other bonsai growers nearby that you can share ideas with?