Are there tips and tricks for SOM for place power pin and reduce PCB layers?
There are plenty of rules, but layout is more of an art. In any case, you have to understand your abilities and out of that to define your priorities.

For instance, if you are a newbie to layout, maybe you should have more layers and less risk of messing it up.

Also define the risks and start from the highes, that you wouldn't be able to mitigate:

High speed first, sensitive analog- next, etc. 

Good placement will save lot's of effort during routing. So think about your signal flow and power flow. Draw arrows on your board to see how you place components. Like input -> buffer -> adc -> asic -> dac -> amplifier -> output. This way you will be able to place everything and see how things connect one to another.

For power consider ready made modules. I don't know how cost sensitive you are, so you may want to reduce at least this risk.