I am extremely new to this and have had a hard time finding information outside if this that isn't condescending. So, first thank you for this website

My niece inherited hermit crabs from her teacher and as she was leaving for boot camp, I took them on. She knew nothing about them. There are 2 males (I now know) I have upgraded their tank to a 25 gallon. They had a sponge in which I replaced with 2 pools (salt water) and fresh water. They have 2 hides and plenty of items to climb on. I used the coconut substrate and there is easily 6 inches in the tank. I did not realize they needed sand too.
My questions are:
What is a good type of sand? It is necessary? Should I mix it or separate it?
There are 2 males currently. Should I adopt another? I read they do better in groups. If so, does it matter the sex?
I had an idea with chia seeds. I was wondering if chia is healthy for them and if any of you have ever build a little chia garden in their tank?
And lasty, any food items that your hermit crabs enjoy eating??
I just want to do right by them. I adore them and am so excited to have them now

Thanks in advance!!!
Jodi