hi, i feed my young emperors 2-3 crickets about 1/4 to 1/3 the size of the scorpion per week. If there is still any crickets left in the enclosure the next day i remove them and try again a few days later. Make sure you mist the enclosure with water every day to keep the humidity at 80% + and always provide a water dish. Im not sure what spidermite is but the substrate needs to be deep to allow it to burrow and needs to be able to hold moisture. Coconut fibre or a pesticide free compost/soil is perfect. The heat mat needs to be placed on the side of the enclosure and you need to keep the temperature in the high 20s.
Your scorpion probably has another 4+ molts before it becomes an adult. Make sure that the enclosure is always humid to avoid any moulting difficulties and dont offer any prey for about a week after it has moulted as its body may still be soft and so will be vulnerable to attack from the crickets.
Have a look at the caresheets section for more info.
If you want to have a good chance of filming your scorp catching prey I would suggest waiting until night time and using either a red light or even better a UV light to see the scorp. Red light doesnt seem to effect the scorpions when going about their night time business but allows you to see them and UV light makes the scorpion flouresce bright bluey/greeney and again doesnt bother them. If using a UV light only use it occasionally and for short periods of time due to the harmful UV rays is gives off.
Hope this helps and congratulations on your new pet.