Ultimately this does come down to what WordPress itself can manage (as our themes are built on this), so any "limits" will be related to WordPress and of course your hosting provider. You can find some useful information about this
here, including hardware limitations etc. Yes you can use multiple themes on the same site particularly if you were looking at installing in subdirectories, e.g.
yourdomain.com <= Your homepage/Vantage
yourdomain.com/classifieds <== Your ClassiPress installation
yourdomain.com/jobs <== Your JobRoller installation
For this you may also want to consider a
WordPress multisite setup. Thanks.