Aircooled:
Basics on amp ratings, cabinet ratings, . . .
First, be sure you do your homework on the Mesa amp: 200w into 2?4?8? ohms? Does Mesa recommend one impedance, or what is the minimum impedance they recommend? Say, 4 or 8 ohms, but never 2, for instance. Mesa maintains a pdf manual library on their website. Do yourself a favor and GO READ IT if you did not get a manual with this amp.
Generally, you will not blow 200w cabinets with a 400w amp. In other words, more amp than speaker is better than the reverse. Why? A small amp pushed hard (too little bass amp fighting Marshalls, drums, keys) will distort as it overdrives. Electrically this power amp distortion (NOT the desired distortion tone that occurs in the preamp) is generating DC pulses to the speaker(s). Speakers want to see AC (+ then -) which push and pull the cone normally. When the power amp distorts into DC, you get only one-way motion at the speaker: This turns your speaker into a heat sink. Let this go on long enough and the speaker will fail. Either over a long period of time, or maybe in as little as a set or two if it's bad enough.
A big amp, running at a smaller percentage of its capacity, will calmly put out clean power all night long. And this clean power is very unlikely to hurt speakers. Of course, you wouldn't bridge a big Crest through a 5 radio speaker, but you get the idea.
Strategy's are one of the most delicious power amps ever made. Congratulations. Most cabinets you are likely to consider will work just fine with it. Paired with an ALEMBIC preamp you would utterly have the market cornered on tone.
About power ratings, incidentally: Remember that sound pressure levels (or loudness) increases LOGARTHMICALLY: If you drive one cabinet off the A-side of your amp at 200w, adding the same cabinet on the B-side (now 400w with two cabinets) will only increase the db level by +3db. Things don't get twice as loud every time you double the power, so don't fall into that particular mental trapdoor, either.
Best Wishes,
J o e y