Those breeds should work fine together.
Speckled Sussex are middle assertive, not particularly flighty, decent layers of generally mid tone beige eggs. Also considered dual purpose as heavier than a Leghorn, but my Speckled Sussex never got particularly large.
Columbian Rock are curious birds, middle assertive, though can be a bit more aggressive especially the roosters, good layers of mid tone brown eggs. Eggs can get pretty large as they get into their second season. Considered dual purpose as a heavier bird than a general layer.
So you have pretty well matched size and temperament, two important caveats about mixed flocks.
If you have an area with a lot of aerial predators (hawks and such), your Speckled Sussex will fair better as they blend into the ground color while your Columbian birds with their white bodies will be much more exposed. (I used to get a lot of aerial hits until I went to darker birds).
As to aging, neither one is the commercial hybrid Red sex links which generally play out in 2 years (often dying from internal cancer by 3 years or some unknown failure). My Rocks and SS were longer lived...but not as long as my mutts who seem to get the best of the gene worlds.
Go for it. Good luck, and let us know when you get your little peeps
LofMc
