i would have to say that a kiddy pool full of good organic soil would yeild much more than planting each plant in a two-litre bottle!
think about the difference in the volume of soil between the two. just because the pool is shorter doesnt mean that the plants wont be able to grow as tall? its the total area the roots have to grow in that affects the plants final outcome. say you wanted to plant ten auto's(auto ak) in the pool, take 10 two-litres of soil and dump them in the pool, it may barely cover the bottom. if you fill the pool all the way up those 10 two-litres may only make up 10-20% of the total volume, allowing each plants root system approx 80% more room to grow.
roots will grow latterally just as well as down. the extra volume the kiddy pool provides will also reduce the frequency of waterings, along with better ph balance, and reduce plant stress. also you could screw on some chicken wire around the pool very easily!
im not hatin on you lehua, im just stating what i beleive to be fact.
aside from the stealth of using two-litre's or other individual containers, i would have to think that a pool full of autos would have more potential despite its height.
ps. planting distance would depend on the type of auto your growing, your probably just going to have to make your own best judgement decided by the plants potential size. after your first run you will then know exactly where you want to plant them(experiance!) make sure that you clean and rinse out the pool before you use it. scrub it with dish soap then rinse, disinfect with bleach then thoroughly rinse and let dry. dont forget to make some sort of drainage holes in case of heavy rains.
good luck