It seems that it is ok to daven in a different nusach, but one should follow that nusach correctly. Maybe there are different malachim per nusach – litvishe, taimani, etc, so you want to be accepted fully by one of them. It makes sense: you can figure out one word by knowing the person’s nusach from the rest.
>> davening is for US
here is an example where capitalization is used, but may be confusing. Who knew that davening can be so America-centered!? on the other hand, Hebrew has no captal letters, so I doubt that Sh’A addresses this