Yes, that's legal. But in many cases, the result could be weaker than a regular monster with the same CR.
As long as they make sense then you can stack a bunch of them...
But as mentioned, you might be better off with a different monster?
Yes, it legal but still need to check the resulting monster vs table
1-1 in the bestiary 1 page 291. To see if resulting monster is the right CR as far as power. It may be way underpowered or way over powered or just right. Remember those templates are just rough guild lines to advance monster they are not perfect each and every time. You should do that check for any monster that you modify be it will adding treasure, adding class levels, and or templates. Also not the more you add / change it form the base monster the more likey that it will off.