Most of what you're asking can be found on a couple of articles on the minecraft wiki site, but I'll summarize the big points:
Villages are specified by the existence of doors that meet certain conditions. For a door to be counted as "in a village," the door must have more blocks on the outside than on the inside and a villager must exist nearby - no more than 16 blocks horizontally and 3 to 4 blocks vertically. A block is counted as inside if it is shaded from above. A block is counted as outside if the sun hits it from above. Here are some common techniques for registering doors.
If the distance requirement for a villager nearby a door is not met, the door can become unregistered after some time. That can be resolved by destroying it and replacing it.
Each door can support .35 of a villager, i.e. you need about 3 valid doors for each villager (more precisely, every 20 doors support 7 villagers).
The boundary condition seems a bit hazy, but the 16 block requirement in the first point seems to imply that it's 16 blocks from the most outlying door. However, villagers can move up to 40 blocks beyond that before they actually unregister from their village.
You must have at least two villagers if you want to start an entirely new village. That means you've either got to find a village and transplant villagers from there, or cure Zombie Villagers using a Splash potion of Weakness, Golden Apples and time.
Once you have two villagers, you can have them create new villagers by "making love" (villagers are genderless, so any 2 will do).
Villagers will attempt to birth new ones when the village can support more than the current number, as per the third point (doors can support .35 of a villager).
Iron golems spawn when there are at least 16 adult villagers and at least 21 valid doors.
Iron golems and villagers both will wander aimlessly if they are not created naturally or are not in a village, respectively.
This information is valid as of snapshot 12w21a, version 1.3.1.
EDIT: To answer the more specific questions OP had...
I don't think anyone really knows, except for the developers, where the hard cutoff for the village is, or if this is one. Apparently at some point, Notch was going to put in some physical boundaries for the villages, but they might not even exist in newer releases.
This mod claims you average out all the locations of the valid doors for the village center, then that the boundary is the largest distance from the center to the farthest house (or 32, if there are no other houses) but it's not clear whether the mod maker thought of these arbitrary boundaries or whether they're actually in the game. I'm going to stick with what the wiki implies and say that villagers register to a village and probably wander up to 16 blocks beyond the outermost doors, and only deregister from the village if they're beyond 40 blocks.
Docm's advice is about the distance between two villages.
Your target is to build one village and another one far enough from the first to be counted as another and not the same village.
As the distance of your villages is 75 and the minimal distance between villages needs to be at least radius+32 Blocks, while radius describes the distance between the first village's center and the furthest door of it, your villages shoul be far enough from another.
There are now two error sources:
Either you have got any door between these two "villages" which connects them to one large, or you are too standing too far away from the not-working platform.
In the first case, destroy the doors in the middle.
If you haven't got any doors in between these villages, just try to play/stand/run whatever between them.
Depending on the servers configuration there is a limit of blocks that may be between you and the mobs (usually about 60-80 blocks). If you are too far away the mobs chunk stops working. This means, the mobs are not running around, not spawning anything and for sure not breeding anymore.
So, to get your farm working properly, make sure that all chunks of the farm are loaded.
Which you may either achieve by being between them or putting one of your friends at he bottom spawn layer and yourself at the upper.
Depending on which plugins are installed on the Server there may be even a block or something called "chunk loader" but one of them at the bottom-spawn-area and your problem is solved.
Best Answer
I think that your doors are not considered as houses for some reasons. Have a look at Tutorials/Village mechanics for what you can change.