Re: Common Login table
I've never tried authenticating a classipress login instance against another
WP instance. I'm sure it could work but the only problem I see would be the ad author not mapping over properly and also a possible cookie security problem.
I don't want to promise you something and then you buy classipress and it doesn't work so here's what I recommend.
Install two versions of wordpress just like you would with classipress and see if you can get the cross authentication working. It comes down to it more being a wordpress limitation vs a classipress one.
Would be interested to hear how your test turned out.