I've been scanning through packages on Synaptic, and found some that piqued my interest - apt-rpm, yum, and alien.
I know of alien, I've even used it once before, but the other two caught my attention.

It made me wonder if, with them installed, could one add an rpm repository to the sources.list (And if so, would it just be rpm http://address etc, or something else?) and what would be the compatibility?

Questions like...
Would Synaptic be able to handle it?
Would apt-get update/upgrade also hit rpm repositories, or would the yum package be needed? And if yum was used, would it be able to use packages from deb repositories.
And finally, what kind of caveats, aside from potential system and dependency problems, would one expect from using an rpm repository alongside deb ones?

No, there is no compatibility.

And for what use ? Ubuntu provides 20,000+ packages,
a collection of rpm repo's maximum 15,000. ?
And most often the same packages.
All the special packages : See Ubuntu.
Which rpm package do you want to install ? ?
.....

I don't know exactly. I haven't looked into any RPM repositories yet.
I had thought though, that with apt installed as normal for a deb based system, apt-rpm would allow RPM ones to be used.
The main reason I asked is because sometimes I find software in RPM but not deb, and I don't always feel like compiling it myself, when Alien usually solves it.