Cool.
What I would do, is get a 3" hole saw and some 8 oz solo cups- smaller than the regular ones. In any case, you need cups that would sit in whatever diameter hole you are making in the bottom of the bucket.
Carefully tip your bucket on it's side, and then use a 3" hole saw to carefully saw through the bucket, taking care to disturb the roots as little as possible. Now that the hole is there, take your cup, poke just a few small holes in it, only a few around the bottom are needed, don't try to make a netpot.
Now, you need to fill the cup to overflowing with promix or whatever medium. Don't pack it into the cup tight, but do make sure that the medium is completely filling the cup, i.e. no air spaces.
At this point, without spilling out too much of the medium, you need to "buckle" or fold the edge of the cup just enough to fit the rim through the hole, then pop back into shape, thus attaching the cup to the bucket.
With that done, take the time to drill about 40 small holes in the bottom of that same bucket all around the large hole. Once all this is done, pick the whole thing up and put it in the outer bucket. Now you can either put a bulkhead sitting in the outer bucket, just below where the inner bucket sits, or just a hole there. It should look like this, but with the cup sticking out and without the fill tube.
View attachment 2101050
From this you can see how the fill tube was put in, too hard to do it once the medium is added.
View attachment 2101051
This is what they look like put together but empty.
View attachment 2101052
Since A: you didn't install a fill tube when you put the original medium in the upper bucket, thus making it really hard to install now, and B: the whole point of this was to make this stuff automated- I would install a bulkhead fitting so you can tie all your lower buckets together to a common drain, then use a float valve or similar to keep the level of water just below the level of the inner buckets.
Oh man! TB's got the 411 on SIPs as well as aero! J/K wicking properly is a concern. That said, I've done SIPs in Ocean Forest, Fertilome, Vic's Super Soil, Store-Brand potting mix, straight coco, and mixes of old soil of any of the above. They all work fine. Most any pre-packaged soil or "soil-less" mix, is going to have a ton of coco or more usually peat moss added in. Plus perlite and lots of other particulates. They will soak up water to the top of the bucket, no worries. Hard to go wrong here.
And you said you used straight promix, which a soilless mix containing about 99% Peat and perlite- it is an ideal medium for use in SIPs and will wick as well as anything else.
Now, if all this just seems too much work- there is another way to solve the watering issue without doing SIPs. You could drill small holes around the very bottom of each pot or bucket, place said pots/buckets in a tray, and flood the tray periodically, to a level just above those holes.
In fact, I keep my mums in square pots that in turn sit in low sterilite tubs. When I water them, sometimes I just dump about 3/4 inch of water in the tray, and the pots wick it right up! I keep mums in straight Ocean Forest, and it works great because the larger mums that need more wick more, and the smaller ones that are still damp wick less. The one thing I don't do is leave any standing water in the tray after about 30 minutes. If there is any, I put too much water in.
Watering from the bottom up is the way to go, unless you're trying to get nutes into a specific plant, then I top water that one.
If you need more help with converting your SIPs, I might be convinced to make a pictorial while I convert some of mine to be on a common fill/drain- I'm gonna do some tomatoes this year in them, and some squash and cucumbers I think, but I'm not making any more SIPs that aren't all watered from one point.
Crawling through the weed jungle every other day to water my 9 flowering SIPs individually is a sweet, sticky irritation!
idiots
... They are currently not for sale at the moment, but if we can learn to get good results with it, I believe he will get back into business with them again. It is somewhat similar to the Atomix, but at a fraction of the price and a few less bells and whistles. The hope is that it can be upgraded and operated properly, and hopefully see some decent results. He lists the nozzles as producing 30 micron mist, which is a tad on the small side, but we can work towards the 50 micron size with a few adjustments. I am quite excited about this, and how an AA system just fell into my lap! 
. I got it from the rvfilterstore.com and they are sending me a new one AND theres a chance i could keep this one as a back up!
Da FUCK??...