Any Planescape adventures/books where the rules are broken?

Well I  think that carefully worded wish would be fine. You wish that you stoped existing in Sigil and start existing somewhere else.

Anyways back to your topic: In one of adventures of our DM we (PCs) managed to destroy one of the Sigil's portals by destroying its arch, wich resulted in tear in fabric of reality that sealed (healed?) itself in couple of rounds by sucking in surounding matter.

I think that later our DM learned that you can't realy destroy Lady's portal, so later we never managed to pull that stunt again.

As of your summoning question: summoning works in Sigil but only if desired creature is inside city when the spell is cast. As a rule of thumb DM should probably look rarety of desired creature in old PS monster manual, or he could use one of many Sigil planar NPCs. In other words summoning is great way to make new enemies out of your planar neighbours.

Pretty sure you can destroy portals in Sigil - in Uncaged: Faces of Sigil there's a character that's made it his life's mission to destroy all of them, IIRC. So, either your DM is wrong, or there's a rule broken

In Torment (which is no book, but an adventure... ) the portal lens and the labyrinth of the modrons seem to be technically broken, as the lens brings you from Sigil to the modron labyrinth and the engineer there can teleport you to any accessible part of the City of Doors. Well, to me that's an example of how you can breake rules to enhance an adventure.

But I believe there was something in a book that seemed odd, just can't remember what it was, at the moment...

I like your reasoning Anime Fan.

And while we are at it let's not forget that Lady's rulership over Sigil was once contested with god Aoskar (presumably). So I guess when she put him in dead book she also destroyed portals to Sigil that he created. Hence those portal stones that nullify portals. Coincidental side effect or Aoskar's last act of defiance towards Lady it is for any one to decide.

Also let's consider this: Sigil's portals are not only ones in multiverse, and many poverful beings can create their own (there is even feat Create Portal wich enables mages to create their own permanent gates).

I'd say they use the manacles and mancatchers to keep their captives from shifting away to other places within Sigil.

The Harmonium presumably put them on so that if they go through a portal it is not an issue. Not only is there the chance of randomly going through one by accident, but also they may feel the need to take people out of Sigil to be punished. Though that would probably be more the Mercykiller remit.

I'd imagine they use those devices more on Arcadia and Orthos.

I believe it was also stated there you can destroy a portal in a less spectacular way. You just need to remove the bounded space. Destroying an archway should do, as well as bricking it up.

True. Compleatly forgot about that. (drat, double post again)

Hmm - I've been rereading In the Cage, and there's one thing that contradicts what I remember reading elsewhere. Here is says that if you fall/are thrown off the edge of Sigil you fall forever. I'm fairly certain I've read somewhere else that if you go over the edge you'll reappear in a random place across the Planes at some point. Anyone know the dark of this?

The important question here is, "why do you ask?"

As GM you can do anything you like and 2E wasn't obsessed with following rules when it came to doing something cool for an adventure. I would not typically allow teleportation into Sigil with anything less than an artifact, and summoning only usually works if it can grab something from elsewhere in the city - but if you need these things to be possible, then make them so ... that is the power and responsibility of the GM.