To answer your question - no, it's not too much. I believe that's what you need, bare minimum, for a 700+. But it's also enough, if done well, for a bigger score too.
I slightly disagree with the above opinions. You shouldn't waste your free CATs just to familiarize yourself with the test. To get an idea of the pattern, the GMAT OG Diagnostic Test is sufficient. Your own plan w.r.t. the CATs is better, IMHO - use them staggered in various phases of the study AFTER a baseline level is achieved in theory and you have had some practice with the problems.
With
MGMAT, you'll also get 6 more tests. Use those as well, once you have done some baseline preparation (say 100-150 questions of each section with some theory revised).
On
MGMAT - focus more on your weak areas. For e.g. the SC and CR is really good but you may not need to read RC. Don't overdo
MGMAT by digging through each guide, each problem (you don't need to solve addition or basic arithmetic problems, for example) etc. but make sure you cover what you didn't know till now.