http://www.icsbb.com/eicsbb/prd_mp1.asp?bignum=13
Cheekrest, body pins, all accessories you want for a SiG552 are listed right there. However, I do not know how you order directly form the site, if at all. You may need to speak with a retailer to order those parts in. Mind you, none of them are restricted, so there should be no problem and any retailer should do it without any problems (given they actually want to).
The fixed sight on the back can be removed. You remove it from the inside of the body, not difficult. I'm using a scope with no special top-rail or scope rings, and the front sights do not interfere with it. Personally if I were you, I'd find someone with scope rings and try out the RDS and see if it interferes, then if it does, you can look for the extra-tall scope rings.