1. In datashhet for 2.7V - 1Mhz & for 5V - 2Mhz is mentioned.
I am operating at 3.6V. What should be max freq at which I can opearte

 

If it isn't spec'ed at 3.6V (or something very close, such as 3.3V), then it is anyone's guess. It is probably reasonable to assume that you can do at least 1MHz. It is probably not too unreasonable to guess that the rating is somewhat linear between the two given spec'ed values, but that is certainly not guaranteed.

In practice, you can probably actually get quite a bit higher from most actual parts. So question (and answer) needs to reflect the context in which it is being asked. If you are making something that is important that it operate properly out-of-the-box and that it be reliable, then use the 1MHz or, if you really need a bit more speed, perhaps risk using a linear interpolation between the two. If this is for a hobby project, then you will probably be okay to abuse the specs quite a bit and can probably expect to get at least 2MHz out of it. If your needs are something in between, then let's bat them a round a bit more.

 

I would add one more comment. Nobody says you have to run the SPI at it's maximum speed. There may be reasons for doing this but they are not obvious to us. My advice:

1. First make it work.
2. Then make it fast.
3. Last, make it elegant.