As San said, you can mix the access points in the same environment. Your other question is much easier asked than answered.
The reason everyone is recommending a site survey are that you will likely end up with fewer access points, fewer coverage holes, lower channel utilization, and better performance. The survey will likely cost you much less then just adding more access points and it will not be a waist of your time.
A site survey encompasses several factors (not just coverage) and due to these factors the results are not consistent from one site to the next.
The average power level of 3-4 does tend to be optimal, but it depends. Getting all of your access points at a power level of 3-4 (with default RRM settings) does not happen without a site survey. If most of your access points are running at a 1, then that is an indicator that you may need more access points, but once again it depends.
Depends .. What grade of wirless ? Was a survey ever done .
"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin
well getting the site survey done is the best option as it will help you identify the quantity of AP as capacity planning is on the major factor in determining the number of AP's per deployment as part from coverage.