From the nature of the question, I'm assuming you're talking strictly about Confederation corvettes (the cats, of course, had the Kamekh and later the Kamrani, and the Nephilim have the Barracuda).
Officially, the only Confed corvette in the entire continuity is the Venture. The Paradigm...well, that's a tough one to classify; it really depends on your source. The Privateer documentation identifies it as a destroyer. Secrets of the Wing Commander Universe classes it as a "capital ship" (which is all kinds of not helpful). One thing that may help set the Paradigm in the destroyer category is the length stat, 350 meters. That's about four times longer than the Venture, so the Paradigm would be a good deal bigger.
Only problem with the 350 meter stat: it comes from SWCU, so its canonicity is not ironclad.
Of course, the CIC News Ship Database says the Paradigm is a destroyer. That's all the more authority that's needed, right?
I've never really seen much connection between the two, honestly; I don't think one is an outgrowth of the other (I'd say on the basis of shape, but then again the two different forms of Drayman are vastly different in appearance and yet they're apparently the same). It's also difficult to gauge which one is better without either setting them against each other or at least an equal challenger. I will say that I never had to save a Paradigm's butt from the cats, so I would gauge it the more capable class.
As far as the Paradigm being left out of Standoff and Saga, you'd have to ask the designers to be sure. Probably because the Paradigm's role in the Confederation Navy wasn't ever defined all that well. Saga is set post-Battle of Earth, so the Paradigm might've been out of service by then. And maybe the Standoff team just forgot about it entirely...