iPad LTE edition and SIM from smartphone I am thinking if it is possible to use my phone's SIM card in iPad's SIM slot?
This because I need the 4G just only very few times, so when it will be necessary to use it, I will move the SIM from my smart-phone to the iPad.
It is possible this scenario?
Yes, this should work fine. I don't know if I have to answer any longer, but you have to check whether your device has a simlock by a provider. Else you should be free to do this.

You could also use personal hotspot.