Yes, shrinkage will definitely be an issue! The metal will contract for two reasons: due to the cooling and, mainly, due to phase changes during solidification.
In order to determine which phases will be present during the process, you use equilibrium diagrams that match the composition of your alloy.
For a given alloy, you will be able to consult literature for the specific shrinkage.
However, it is not as simple as just scaling the CAD model accordingly! Because the shrinkage will not be uniform, as the temperature gradient within the mold is different, the last liquid metal to solidify won't be fed further liquid metal to compensate the shrinkage, forming a defect. To avoid this, you need to use feeders that solidify last and provide liquid metal to the part you want to obtain. For this to work, you need to determine how many gates, feeders, including their volume and position, you need. This is done through archaic methods, experience or through simulation software like ProCast, SolidCast, MagamaSoft, etc.
Finally, modelling a part for foundry needs to take into account the draft angle and excess thickness for finishing, normally milling faces, rectifying cavities and drilling holes.
If you have no idea what you are doing, consult a metallurgical engineer or a foundry.
Aluminium can be cast in low pressure (sand or metallic molds) or high pressure (metallic molds - similar to plastic injection). Both processes are quite distinct and some parts are only suitable for one of both processes.
Regards,
Vitor