You can do that, but you should be aware of several potential problems (some of which you already know, it sounds like).
1) Skipping the essays can lead to artificial score inflation on the later multiple choice sections.
2) Good CATs are hard to find. If you "use up" the good ones doing only partial tests, then what are you going to use later on when you want to take full tests?
If you're struggling to find something that will help you gauge quant performance in an adaptive, timed setting, try GMAT Focus (
www.gmatfocus.com). These are mini-quant diagnostics (24 questions) from the real makers of the test, but the question don't overlap with OG12 or GMATPrep. Adaptive, random question mix, the whole thing - they're really good for practicing timing and gauging your progress.
I think it costs about $25 to $30 per set of 24 questions, and I think they guarantee either 3 or 4 sets of "clean" questions (no repeats). After that, you might see questions you've seen before.