Longstreet does have minor basing requirements:
Infantry and cavalry bases should be either square or a shallow rectangle (3:2 width:depth ratio).
Artillery bases should be the same width as infantry and cavalry bases, and as deep as required by the model.
You cannot go far wrong with following the Fire and Fury guidelines above – the rules state you can vary depth by 1/8 inch either way (the width must always be 1 inch). So you could mount infantry in 1 inch by 1 and 3/4 inch or square if you want to.
In my experience, the physical size and shape of your figures determines the number of figures per base and whether you have single or double ranks.
8 figures is possible if you use a smaller scale figure than intended for the base. I have two armies of Heroics and Ros (5mm) and Adler with a few Baccus (6mm) – all are mounted on bases meeting the 15mm Fire and Fury standard.
The 5mm Heroics and Ros infantry are in two ranks of 8 or 9 figures (first rank of 3 or 4 in skirmishing/firing pose, second rank of 5 in marching pose)
The 6mm Adler infantry are a single rank of 5 figures in marching/advancing pose
Both sizes of mounted cavalry are a single rank of 3 figures
For dismounted cavalry, 3 Adler or 5 Heroics and Ros placed unevenly.
Artillery is a single gun and crew (you get larger crews with Heroics and Ros)
I chose for 15mm standard bases to ensure compatibility with other player's armies (you have to ignore the disparity in size), and because it's easier to pick up the bases without touching the figures.
For 15mm, the consensus at our club (GLC Games Club) is 3 figures in a single rank for infantry, 2 figures for cavalry (both mounted and dismounted), 1 gun and crew per artillery base.