Last question and I'll leave this alone and accept it as something I'll never understand but can use lol. Does the outflow pipe from the siphon pipe at the level of the sump stay under water or above the water? I want it below the water level to cut out splashing noises.
It doesn't matter, the siphon will start either way. It'll start faster if the bottom of the drain plumbing is ABOVE water level, but it'll be noisier that way. Personally, I like to have it terminate JUST below water level. Very low backpressure so you won't have starting problems, and it's submerged so it'll be quieter.
I think I've grasped your confusion - are you used to talking about losing/starting siphons in a hang-on overflow that has a U-tube siphon over the side of the aquarium? That's a very different animal than this setup. With that configuration, you are correct, you never want the siphon tube to have air in it. If that sort of siphon gets air in it, it won't restart on it's own, because there's no "motivation" for the water to start flowing UP above the water level in the aquarium to fill the siphon tube and restart the siphon. With this Herbie configuration, it doesn't matter, because the entire run of plumbing is BELOW the water level in the aquarium. So when you restart, it completely floods the siphon, and it starts sucking away.
How does the locline returns work? Can I just attach them to the bulkhead or do they have a special connector?
Most locline kits will come with a threaded adapter (male or female) that will mate to standard pipe threads, so just determine the size and orientation and you'll know if it will work without another fitting.
Do the ABS bulkheads fit with the regular schedule 40 pvc pipes?
Yes, more or less. The ABS bulkheads common in this hobby are made dimensionally compatible with standard PVC. The one hitch is that if you get a bulkhead with a slip fitting, you will want to make sure you get a cement that's rated for PVC and ABS, as most common PVC cement isn't technically rated to bod with ABS (it'll work but it won't be a great bond).
You probably know this, but when you turn the return pump off, the water in the tank will start to siphon back down the return plumbing through the return pump. Essentially, the tank will empty down to the point that the opening on the return plumbing starts sucking air. Because of this, you will want to keep the opening of that locline high up in the water column, or have some provision for a siphon break.
Does a mag drive 9.5 seem reasonable? The 7 seems a little bit too perfect with 400gph at 5' head in case I ever want to put on co2. The 9.5 is listed around 720 gph at 5' head
Personal preference. It's probably better to have a larger pump than you think you need. No matter what, you should put a valve between the pump and the return so you can dial in the flow.
edit: apparently, for bean animal, I'd have to get the 18" long box lol hmmm decisions decisions. that extra safety pipe would let me sleep better at night
I know some will vehemently disagree with me, but the Beananimal configuration always struck me as excessively way over the top. I've run lots of tanks with "just" a Herbie and have never had even a shadow of a hint of concern. But to each his own!