Three weeks might be a bit old to put day olds with. You could partition off and put some on one side and some on the other to get used to each then see how they do.
As long as there's room, quite a while. How old are the chicks your putting with chicks? I'll take day olds and put them with week olds and keep doing that every week for about a month. So will have month old with day olds and no issues but they have always been raised with younger chicks, so...
What kind of incubator broke? If the leads going to the heating element are putting out 110v ( they should be) you can just hook a light bulb to that. Just take the leads and wire them into a lamp base and screw your bulb in. Lamp bases are about $1.50 at lowes and places.
A while ago when we lived in a town up north, I was good friends with zoning. He said no farm animals, it had chickens in there. We had three lots with our home, so me the way I am, built a huge pen and raised 100 pheasants. Lol every complained, but there was nothing the city could do.
Yeah don't use radio shack. Do you need that part in the pic? That's a male PC inlet power socket, Amazon or frys will have. If you just need the cable that plugs into it, anyone will have that. Most tv and monitors use them.
I tile everything, my buddy works for a flooring place and they have 1-2-3 boxes of tile left over from jobs multiple times a day. I also run one tile up on the wall so if I ever wanna do a wash down I can. Ask around, you would be surprised how tile you can get for free.
The vent is so you won't loose hot air unless you want to. Maybe go with a smaller fan, the might be pushing around more air than what the lights can keep up with plus it's pulling in ambient temperature that could be fluctuating. All I use for my refrigerator bators and the ones I make are two...
Nice looking setup! The fan pushing in, when does it kick on? Is it for cooling? If not you may wanna turn it around and add some (dryer vent) louvres to stop it so your bator does not thermo cycle itself when nothing is on. Just keep in mind the thermo cycle thing, a lot of air can move around...
Is staying to hot? You may want to cool it also. That's my biggest problem in tx, gets too hot. Cut a hole and mount a fan blowing air out of the bator, hook it up to another thermostat set at a higher temp.