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I am not mechanically knowledgable, and I'm wondering if my mechanic is trustworthy.  He recently replaced my master and slave cylinders (which were only 1 1/2 years old) when my clutch stopped functioning, and as I drove away the car was hesitating in the lower three gears, then leaping forward.  Then the last time when I floored the accelerator, nothing happened until it suddenly got REALLY loud and went forward - after that it sounded like I had no muffler.  I drove right back, and he looked and said the pipe connected to my muffler had rusted and I needed a new muffler (he showed me and I could see the break, but not well because only the back was jacked up a little).

Then he said it was probably from the catalytic converter not being in perfect shape (he knew I was concerned about it after driving on lots of bumpy roads, and smelling sulphur a few times awhile back).  He said if the converter isn't working right it allows moisture to pass through to the muffler, which will rust it out sooner.  Then, seeing how pissed I was and how skeptical, he said he didn't have the necessary parts and referred me to another repair shop who he had and they were going to charge $300 and change.

Not trusting him, I drove around the corner to a little exhaust specialty shop and said my car just got real loud.  That guy also identified the rusted part, and said he could fix it for $125 but not for a couple weeks (ordering & vacation).  I asked if it could have anything to do with the catalytic converter, and he said no.  He also said he saw nothing that would suggest there was a problem with the converter.  He's an exhaust specialist, so I am opting to trust him over the first guy!

Now while I'm waiting for guy #2 to return to replace my muffler, my car is stalling out instantly after starting, and when I eventually get it moving by giving it lots of gas, it will run ok in higher gears for the most part, but it is hesitating a lot intermittently.

So my questions are 1) is mechanic #1 totally BS'ing me about the converter?  and 2) could the current stalling be just that the muffler is getting clogged, or is there another more likely explanation?  

A clogged cat can cause hesitation.  An exhaust shop should be able to test for a clogged cat fairly easily.  Your hesitation could also have nothing to do with your exhaust.  Let us know what you find out. Or if you have any other questions