Let answers a couple, I do not have vents, but they about the same as imitators.
I'd pick up a 15gal high, they are very cheap, but offer the heigth without the trouble of sealing a 20g on end.
My guess is that 4 would be fine in a 15gal high. I currently have 2 and they do not use the whole tank all that much.
I think they can breed very young, as early as 5 months.
Just dump the flies in they will find them. Mine are not on the ground that much, but they have no problem finding food in the tank.
I do not use any broms in my tank, but I might change that. I had a few people tell me not to use them, and a few tell me to use them. I like my tank, but they are hiding in places that are very tough to find them.
A couple of my suggestions:
1. use ver fine gravel, my imitaors like to move the larger gravel and then hide down in it. I almost have this stopped. I am going to remove the plant they are doing this under, for fear of one of them getting stuck under a rock.
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Kyle
1.2.0 D. leucomelas
0.0.2 D. azureus
1.0.1 D. imitators