can I use 7/8" gravel as aggregate?

I'm adding a support pier under cottage to prop up a beam mid point (advice from others here on an older / previous post re: bouncy floor).

I'm using a 28" Bigfoot footing form with approx 2' of 10" sona tube at the top - down to about 4'. All together I think I need about 5 cubic ft of concrete to fill it (3.5 for the bigfoot, according to the label, and estimating another 1.5 for the tube). Portland cement bag label says it covers approx 15 sq feet for a 4" slab which is approx 5 cu ft.

I have a pile of clean gravel (7/8" max sand/gravel mix) from a local quarry that I had dropped off a year ago. I'm fairly remote and getting a truck out was big $$$. So I'm thinking of just using some of this gravel to mix in the concrete as the aggregate. I'm hoping that as long as I use the right water / cement ratio and drop a couple pieces of vertical rebar in the tube I should end up with a decent result.

I don't want to go crazy analyzing the choice of aggregate or go insane worrying about 5 ft of concrete - but I also don't want the pier falling apart. Any thoughts on this? Has anyone seen a small concrete job go bad because they didn't cherry pick the sand and stone? Also how long should this thing cure before I put any load on it (assuming it's curing underground - am I correct in assuming this will be a while longer?) This will end up being one of 9 piers supporting a 20x20 cabin.

Thanks