I'm not sure you will get a desired effect. I have a Tetra bottom drain. It is a 3" that I foolishly plumbed in 2" to the mid level input on my skimmer. (It's the biggest mistake of my pond build) I do get water draw and I'm very happy I put it in, but the amount of crud that freely chooses to go to the bottom drain is minimal. It works well as a in pond vacuum when I use a metal hose extension to flush debris into it with the pond full of water. So I'm glad to have it.
The issue with the retro BD is that you won't have the benefit of gravity as stuff has to rise to the outlet rather than fall into it. Secondly by plumbing it to your skimmer you loose the direct water draw through it because your skimmer weir has much less resistance to flow. You can't separate the draw to the BD when it's plumbed into the skimmer. I close my weir door by hand but the BD doesn't draw enough to feed my pump.
I would strongly suggest you plumb the BD to it's own filtration unit—55 gallon barrel or? The BD will provide a great deal more glop to your filter. If your skimmer has adequate filtration that is good, but you'll need to clean it very frequently. If plumbed separate this sets you up to later do an external pump. In this way you have some control of the draw. The pump could go in your settling unit for BD and plumb the skimmer into it if you only want a single pump set-up. This way you have full draw on the BD and can valve down the flow from the skimmer. Essentially a skimmer doesn't need settlement but doing it this way saves on an extra pump.
It would also be beneficial to use larger diameter pipe for your BD. The aqua art is one I've heard recommended before as it will accommodate bigger diameter pipe.
Gravity flow for 2" is 750 gph, 3" is 1500 gph, 4" 3000 gph—
http://www.aquaart.com/pondFLC.htmlI have six brushes and two layers of mats in my skimmer/BD filter. It loads up pretty quick this time of year.
