First, I make all of my parts, paying close attention to front, back, right side, origin, etc. like we all should.
When creating a new assembly, starting with the base (fixed) part, how do I get the origins aligned?
I have been doing this via mates to the three planes (or use the origin) between the new part and the new assembly I am starting to create. It would seem to me that the first part should automatically be aligned to the origin (and orientation to the front, side planes, etc.) of the new assembly when you are creating the new assembly from the first part???.
The part "front" plane does not necessarily have to line up to the assy "front" plane - you can rename the planes to anything you want. I think you are trying to simplify something that isn't necessarily easily done - let me explain what I mean and how I try to train others.
Sw gives you the opportunity to drop a part into the FT of an assy and then it will align it with the assy planes and fix it in place. Quick & easy, no doubt, and at times, that's an acceptable method.
But I try to impress on others that there might be a reason to not have it there. If you look ahead a bit at how you will be using the assy, and that part in particular in relation to the other parts in the assy, you might realize that some feature of it might come in handy later if it were positioned a particular way. For example, you might model a part with the system plane at the bottom of it based on the geometry of the part. However, 2/3 of the way up there is a hole in it that will have bearings and a shaft associated with it. So, to facilitate the assy model's relations to the shaft, you may want one or two of the assy system planes to go through the center of the hole, and this can be accomplished by using the temp axis in the hole.
Another reason to avoid the "fixed part" approach is especially important if you are not the only one doing the modeling. If you drop the part into the graphics area of the assy, rather than the FT area, it will nail it down right where you drop it. Maybe close to the system planes, and then again, maybe not. Or even worse, a co-worker does it and drops it pretty close, but not right on, and then you get it and use the assy system planes to locate the assy. Then later you scratch your head trying to figure out why bolt holes don't quite line up. Can you really trust everyone to do it correctly? If the part is just "fixed" rather than mated to planes, you don't know for sure which way they did it.
By aligning the part to the planes manually, it makes you take the time to consider how this part really needs to go. You may not know for sure, and its orientation is changeable, but at least you put a bit of thought into it.
Another general rule of thumb that I preach is to always use midplane extrusions & symmetry for parts, unless you have a specific reason not to. I don't know how many times I have seen models created where the person has the system planes coincident to flat faces, and then has created extra, new planes coincident to midpoints on edges. If you have a reason to put them on the sides, etc, do it, but make it a conscious decision, rather than the default because it's the easiest. So many times you will come back and line up parts by their centers, etc. If you need the outer edges or faces, use them - you don't always need to use planes.
Long reading, but I hope it has opened your eyes a bit.