Getting back to OP's original question-you've got me confused. I could swear I read "it's about 18" to 24" deep......" and then just now, I see where you wrote that the rock is 6"deep. It's entirely possible that I'm misinterpreting something but what i really wish to comment on is your-OP's-misconception about how tree roots grow. To grow outward is the normal situation and most definitely does not lead to a weak or infirm root system. Plant roots-like all plant parts-need to undergo respiration, a process which requires oxygen. Down deep in most soil types, there's little if any oxygen so we don't tend to find roots down there. Whaas already covered this for you so I guess I'm repeating. But the main point is-depending on the actual depth of this rock-there is no particular reason not to plant on top of it. If that poor tree depended on sending roots downward into that tiny area, the game's already lost. The good news is that it is not dependent at all on this. edited to add: Sometimes it would help to read the date of the initial post!

There is I believe a huge difference between baby blue and baby blue eyes. The latter I have NEVER been able to see let alone purchase. Baby blue seem a lot more common. But when one goes to Google or "Ask.com" or even tree websites I will bet big money the wrong answers come up for the description of the different type. So there is a lot of confusion out there between the two even amongst so called "experts". As a result I have no clue about the difference. I have planted baby blue which are much bluer and much denser and more uniform in presentation than regular blue spruce. How different they will be in a few years - compared to the baby blue eyes - who knows.

I would approach the question of which tree to plant as "which conifer can handle a reasonable amount of shade (but that a good number of hours of direct sun wont hurt), that also looks like an Xmas tree". Upon first look that does not appear like an ideal situation (it obviously isn't, that fence height is a good % of the final height you want your tree to be), but as mentioned earlier it appears to get full sun in the later afternoon which have the strongest rays. Having said that, I think it is more than workable to get something that you will be happy with. My experience has shown that norway spruce are a reasonable amount easier to grow that colorado blue spruce(handle shade better, etc). Now you are faced with the dilemna that 50% of the questions on here revolve around (I needed a (row of) tree(s) yesterday that is 20' tall, 9' wide already and wont grow anymore). If you are willing to wait 20+ years then go buy a small reasonably priced smaller (slower growing) conifer, be patient and wait (eventually it may/will grow bigger than you want, perhaps much bigger than you want) Or (if there is even a nursery around you that sells older (and larger) slower growing conifers, you could shell out big big big big bucks and get something bigger so you dont have to wait as long. What Im learning about this hobby is that it is a lot about being patient(or having deeeeep pockets, but even then still being very patient). Im not the best person to recommend specific trees, not much experience. Ive got a lot of basic conifers- norway, colorado, black hills, serbian, canadian hemlocks, etc but not so many fancier/slower growing ones (and Ive only been into conifers/trees for 3 years) Canadian Hemlocks like shade but growing them can be finicky in Ohio(and they may not have the Xmas tree look you are looking for). Actually that is way too much sun for a Canadian hemlock(at least in Ohio, up North not as much, but I would say still too much sun, even where you live). Im thinking some type of slower growing Norway spruce, but like I said, not an expert. Im sure others will offer specific suggestions. The Norway spruce is the most bullettproof conifer I have met. Ive got a bunch planted in shady areas. They dont fill out as much when in shade but they still look great to me and plenty "Christmas-tree like".

conifers are trees .. they NEVER stop growing ... use the latin name to get an annual growth rate ... and just start multiplying after that ... most size estimates are at 10 years.. and if they are honest.. they are ten times the annual growth ... clay soil are not a problem... its all about how you plant in them.. and how you water for the first few years to get them going.. see link ... bad clay does not allow drainage.. so problems happen ... i had tip blight problems on some types of Js ... i am not sure if sky rocket was of the type ... https://www.google.com/search?q=sky+rocket+juniper&ie=utf-8&oe=utf-8#q=juniper+tip+blight ken https://sites.google.com/site/tnarboretum/Home/planting-a-tree-or-shrub