For a 20 gallon tank, yes, you'll want to add a 2 liter bottle with a yeast mixture along with the Hagen system. Two 2-liter bottles may be even better, but you can try to use the Hagen along with one 2-liter bottle.
Diffusers are sometimes tricky with DIY CO2. (I say that because I'm not at all the DIY type!)
The ladder was too big for my 5 gallon tank. It fit in the tank, but took up a fair amount of room. I use two different diffusers (I have two 5-gallon tanks, each with their own Hagen bottle.) In one tank I'm using a Kordon Mist-Air sintered glass airstone, the "fine" variety. (It comes in fine, medium, and coarse.) This airstone is wonderful! However, I have purchased more of them, to put in the other tank, and to have a spare while one is drying out occasionally. None of the others work - just this one Kordon airstone.
In my other tank I have a Rena microbubbler airstone. It doesn't work as well as the Kordon and only gives me about 10 ppm CO2. I'm still trying to figure out what to do for this tank. There are lots of diffusers online - that little glass one that looks like a little pipe - is that the one you're talking about? I called DrsFosterSmith to ask about it and they said it only works with pressurized CO2.
I also used a Red Sea 200 diffuser. I bought one, it worked, so I bought more and the new ones did not work! Ugghh! I stopped using the one that did work for now - it was so hard to "open" for cleaning.
So, I'm in the same boat as you when it comes to diffusers - I'm still trying to figure it out. I would suggest, though, that you add a 2-liter bottle of the yeast mixture like you were thinking of doing.