If the results of the upcoming British General election were to be a
hung parliament with a large Lib-Dem showing (lets say they got 100
seats),
then they would be able to form a coalition goverment? Yes /no??
And they could choose which party to go into coalition with ( Tories
or Blairites) ?  Yes/No ??

And been part of a coalition goverment, they would jointly rule with the 
party of their choice? i.e. they would have a share of the ministers
in the cabinet?

They could (i.e. Lib Dems)  introduce Proportional Representation as a method
of voting in the UK? 
Cheers
Mongolia

Unlikely!

The Liberals were a minority party in 1924 & supported a Labour
Government ... and did nothing.

It's all talk!

In theory parliamentary systems are more flexible and able to respond
quickly to events than the U.S. system. But coalition governments
almost never take decisive actions. They lack an overwhelming mandate
from the voters for bold deeds.

Hi, Mongolia - and Tibet, China and Siberian Russia :-)

Your Lib-Dem's would be able to join in a coalition, but the impulse
would come from the stronger of the main parties, Tory or Labour. In
Germany, we have lots of experience with this, and Badger is right,
especially if the competition between the major parties to form a
coalition was rather even.  The coalition agreement, a sort of
contract, between the parties would define the principles they agreed
to, also concerning the distribution of ministers.  The harder the
competition, the more detailed the agreement would be, with a dilution
of significant election promises.

But we can be sure that the major party in any coalition would never
agree to the introduction of proportional representation, because that
would undermine its own position.

Twenty year ago, The Economist magazine flirted with the idea that pro
- rep was good, but it seems to have learned better, maybe from
observing the situation in Germany.