21 autoflower guerilla grow

Hello all. This season I plan on growing 21 auto flowers. The strains consist of 7 auto white widow, 7 auto super hash, and 7 auto anesthesia. These were purchased from Herbie's and pyramid seeds are the breeder. Here is the rundown, and I have some questions aswell:
The plants will be divided into two plots. One will have 2 different strains, containing 14 plants in total. The other plot will have the other 7 plants.
I will be growing in 3 gallon pots and directly in the ground. Medium is MG moisture control and perlite.
Questions: I am using soil from last year's grow which still looks nutrient rich. Should I fertilize it anyway before planting or fertilize a few weeks after planting when the plant is more mature?
How liberal/conservative should I be with fertilizer, if using any at all?
(Pic is plot one. It can only fit 8 in the ground now. Goal is to fit 12 in ground along with 2 pots to equal 14.)

I did some autos outdoors in 5 gallon buckets with the same soil and perlite. I held off on any nutrients for the first few weeks. Basically until they start flowering. I did the auto ak from gragrassomatiI got 2oz from one and about 1 each from the others. They grow short and bushy, And are done before the photo plants are even budding.