1 1/4" pvc isn't very common, i'd go with 1.5" that's what I did on my 120G in the photos above.
you can get bushings to reduce the bulkhead size, though. I'd sooner just make it 4x 1.5" bulkheads and call it a day, and you can reduce the 1.5" down to 1" with a bushing.
You probably don't find much about internal weir bean animal setups because you can't actually drill the bottom of most tanks, the bottom is tempered glass and it would shatter. On "reef ready" tanks, they drill the holes before they temper the glass, that's why you hear people talking about existing holes.
Only on custom built tanks can you have bottom holes drilled wherever/however you want, and that's why it's not a very common thing to see people doing, as many people are drilling an existing tank and not having one built from scratch.
The other thing about that is, most of those reef ready tanks have 2 separate weirs, usually with 1" and 3/4" bulkheads in each weir, so people will use the 1" for the emergency/trickle and the 3/4" for siphon and return, but you have to have this balancing act of putting the trickle on the opposing weir from the siphon, and then dialing the siphon back so that you still get water into the 2nd weir with the trickle line - to keep both weirs going at once....
If you're going custom, then you can put all 3 drains inside of a single, larger weir. That would make the whole thing much easier to tune and setup in general. However, you're pretty much talking custom all the way now, since nothing like that is really "off the shelf"
you can get bushings to reduce the bulkhead size, though. I'd sooner just make it 4x 1.5" bulkheads and call it a day, and you can reduce the 1.5" down to 1" with a bushing.
You probably don't find much about internal weir bean animal setups because you can't actually drill the bottom of most tanks, the bottom is tempered glass and it would shatter. On "reef ready" tanks, they drill the holes before they temper the glass, that's why you hear people talking about existing holes.
Only on custom built tanks can you have bottom holes drilled wherever/however you want, and that's why it's not a very common thing to see people doing, as many people are drilling an existing tank and not having one built from scratch.
The other thing about that is, most of those reef ready tanks have 2 separate weirs, usually with 1" and 3/4" bulkheads in each weir, so people will use the 1" for the emergency/trickle and the 3/4" for siphon and return, but you have to have this balancing act of putting the trickle on the opposing weir from the siphon, and then dialing the siphon back so that you still get water into the 2nd weir with the trickle line - to keep both weirs going at once....
If you're going custom, then you can put all 3 drains inside of a single, larger weir. That would make the whole thing much easier to tune and setup in general. However, you're pretty much talking custom all the way now, since nothing like that is really "off the shelf"