They're both very helpful so yes but always study the angel/demon you are going to summon,find out their likes,dislikes,days to avoid working with them,insense,candle colour,persnonalities and what they can help you with
It is unwise to summon them without a good reason and proper respect and preparations unless it is the kind that likes it. An alternative to that is just reaching out. By just thinking of their names and saying their names, asking questions out loud about them, researching them etc. you are reaching out. I would wait on a sign for when it is alright to summon them which can happen when you reach out.
It depends. If you think you are not ready and you are afraid of that, then don't. If you think you are ready, start. As ToxicDraton said, you should study the entity before you summon it. Angels don't see humans very kindly... The chances of hurting you are more than demons do.
I believe you should start from demons. If you want any more information, mail me.