Hello Juanan,
1/ COMPRESS does not offer option for design of rectangular vessels. There are no current plans to include this feature in COMPRESS.
2/ Yes, you can design "single" elements. You can design components such as heads, cylinders, and transitions, etc. Flanges designed according to Appendix 2 may also be designed as the "single" element in the COMPRESS file.
Tom Barsh
Codeware Technical Support