In answer to your questions - the people are great, because you work with these people all the time you tend to form really close relationships with everyone, because its such a big family environment everyone looks out for each other and you'll find most people get on really well, your White Star group will probably be your best mates and also you get little groups of people from the same nationality all together, but because your all in the same place you all socialise together anyway.
There are some Romanians onboard, i'm not sure how many and they were all friendly when I was on there, you'll find people will come up and introduce themselves aswell.
As for security, you will be sharing a cabin with someone so as long as you trust them all your belongs will be fine, as you are the only people who have keys to your cabin. You can always hire a safe box from the crew office, like a little box to put cash etc in, and they are kept in the crew office you have one key and the office the other - both keys have to be present to open them.
Hope this helps!