Nick -

Welcome to the wonderful world of 1A2!

1- I guess it would depend on the individual PBX. I am unfamiliar with the Panasonic KX-TD1232 but basically the big questions to ask are: what voltage does it supply for analog extensions? Anything from 24 to 48VDC ought to work well. Then there's ringing current. 70-105VAC at 20hz is what the system is looking for.
2- The usual limitation on phones, especially with a small system like the 551 is the number of lamps that can be lit at the same time. I don't remember what it is for a 551, but you should have no problems with 8-10 sets.
3 - Intercoms come in two basic flavors: manual and dial. A manual intercom is designed for 1-2 users (though more could be added). It takes a card in the ksu and supplies talk power and an in-use lamp. Signalling is done by converting unused buttons on the set to non-locking and using them like doorbells. External buttons could also be added and spare wires in the set are used to connect to the signaled set.
A dial intercom is a separate piece of equipment that connects to the telephone sets. Usually they are single channel units (though multi channel units were produced). You went off hook on the line associated with the intercom, the lamp lit and you dialed the desired party. Signaling voltage was sent on a separate pair to the appropriate set. The only thing the intercom connected to the KSU for was power (Talk [usually filtered -24VDC], Relay [unfiltered -24vdc] lamp [10VAC] and signaling [either 18 or 10vac or ringing generator]). all other connections went directly to the tel sets.
To answer your question - yes, there is documentation, but it depends on which intercom you're talking about.

Hope this helps - at least to get you started.

Sam

"Where are we going and why are we in this hand basket?"