Hello and welcome. I understand your confusion, bonsai soil is the subject of much discussion and it seems no two enthusiasts agree 100% There is good reason for this though. As you noted different species prefer different types of mediums. There are other factors as well, your location and how often you are able to water are a few.
Can I use a basic multipurpose compost and add grit to it for drainage?
Many growers follow a similar plan, creating a base mix and adjusting it to suite a specific tree. I tend to use a similar mix for most everything with a few exceptions. An Azalea, for instance, has a little coarse moss added. Other than when I layer something this is the only time I use any moss at all.
Junipers and Pines prefer a 'drier' mix that drains exceptionally well and retains less water. I have a Juniper in a nearly 100% inorganic mix and Pines completely free from any organic material.
Be careful, if you are going to purchase your medium/compost I have seem some ready made products that are very heavy on the organics. Also I'm not sure what you have in mind when you say 'grit'. Try not to use materials of differing sizes. If your 'grit' is smaller than the other particles it will filter between the larger ones and impede drainage.
Have you read our suggestions on soils and re-potting?