there is rarely just one single way to develop bonsai so you may get different , and maybe contradictory advice. That just means there are many different possible ways.
At this stage soil mix is not critical.
Most of us use a mix with larger particles which helps avoid root problems if you water a bit too often but that can wait until they are bigger.
The pots look large enough for your trees for the first year so they can just be left and allowed to grow. If you manage to keep them alive until next spring we can look at next steps.
Another option is to transplant some of them into slightly larger pots. That is usually done when they have some first leaves like middle right but can be any time from germination through to several months old. I take that opportunity to reduce the 'tap' root to promote more lateral roots that will eventually form the nebari.
Not sure where you are keeping them but lots of light, especially some sunlight is required to maintain compact growth.
Seedlings do have a store of nutrients from the seed but regular fertiliser will give you better growth. Type and brand of fert is immaterial as the plants can't read the packets. All they require is nutrient and all decent fertiliser has nutrients that plants need.
Vachellia is not a species I am familiar with. I had to google to find it is an African 'Acacia' Some of the South African members may be able to give more specific advice on care and styling options.
Adding a location to your profile is really helpful as people will be able to offer advice suited to your specific climate.
