I've already done a lot of research exactly for this purpose (a Grappling PrC).
Here's some work:
http://tech.nhsacramento.com/jwarner/grappling.htm
and
http://tech.nhsacramento.com/jwarner/mighty_contender.htm uses them in a Prestige Class.
Most of those grappling feats are a collection of (IMO) the best grappling ideas I've collected from others on these boards over almost 2 years.
The Sumo has some good ideas, but... I don't know.
Collectively, it doesn't quite 'work' for me.
Maybe he should get his CON bonus as a bonus to AC (like the Forsaker), since his AC is going to be SO bad otherwise.
And the Engulf and Pounce are a good start at making specialized, custom grappling abilities.
Unless you do that for a Grappling PrC, you are only using the rules as written (and the rules as written are a good start, but don't nearly go far enough for a Grappling PrC)
I really like the flavor of the class, and the names are hilarious.
I'd suggest looking closely at Oriental Adventures for Grappling ideas - their Improved Grapple is probably a better fit than your Improved Grab.
Actually, looking back, I think the Sumo is a workable class, probably does what it wants to accomplish, but I think doesn't go far enough with custom grappling rules to make the disadvantages and requirements of the class pay off.
Good job, overall :thumbsup: