Realms, cross play and add ons

Me and some friends are planning to start up a new server and I have some questions…

1. If I create a new world with some add ons (best on bedrock looks cool) do we all need to buy it, or just me?

2. I haven’t played since bedrock got the shader add in, does this work in realms?

3. Will this all work with 2 of us on Xbox and 2 of us on switch?

4. Any tips about all of this?