At Mission Bay Yacht Club in San Diego to official rule is any class with three or more boats can request their own start. In reality, all multihulls usually start as one fleet and sail on corrected time. We all realize the limitations of the rating rules, and the fact that most race comitees do not include the wind strength adjustments. Even so, it is a good excuse to come together and go sailing. When we run a larger regatta, we split the fleet for spinnaker or non-spinnaker boats. This seems to even the ratings a bit.
Your 20 should be faster than the 16's in all conditions. The tigers are faster around the course in theory, but not always in practice. As the boats become overpowered, the Tiger's spinaker becomes less of an advantage.
The fastest way to come up to speed is to invite one of the "A" fleet sailors to sail with you for an afternoon. Switch off skippering for a while and crewing for a while. There are a whole bunch of little tuning things that make the boat go faster, but 90% is basic sail trim, boat trim, tacking and jibing. That last 10% is the hard stuff, the stuff that makes the "A" fleet guys fast regardless of conditions. Some boat tuning, mostly reading the conditions and responding. Don't worry about that stuff for now.
Concentrate your efforts on big picture stuff. How do I get on and off the wire(practice)? How do I tack smoothly and excelerate out of the tack(practice)? How do I round marks cleanly(practice)? What is the basic set up for the boat (rake, rudders, dimondwire tension, rig tension, jib lead placement, downhaul, jib teltail placement)? How do I work better with my crew/skipper (communications, division of duties)? Most importantly, basic boat speed (watch your telltales), and starting near the favored end in clean air. Given the choice, clean air is more important.
Special note to Dan, I just couldn't resist. This was from this, the year the spinnaker the size of Texas was left at home (masthead with a 17' pole). It was all playing current and a clean start next to the rocks. Remember, I rounded "A" mark directly behind the "D" class cat. Then it was all I could do to hold off any of the spinnaker boats until the leward mark.