you can certainly integrate the two langs.
if your program (+depends) is large, then Xcode can get tied up for a number of reasons.
personally, i have had not been able to use Xc4 as effectively as Xc3 on large projects. i spend too much time waiting for it to become responsive. i have loaded large (meta) projects in Xc4 and let it parse and create several GB of 'derived data' for hours (literally) on an otherwise idle 8 core machine -- only to end the process several hours into it. again, it depends on the size of the projects, and their dependencies. it's unlikely that your projects are that complex.
the tradeoff is that Xc4 adds some significant features (e.g. superior parsing, compilation during editing). sometimes it's good to launch into Xc3 and minimize functionalities (e.g. code sense indexing) until it builds for you.
compilation should not cause a freeze (but give it some time to recover)... Xcode may be overwhelmed by errors if you have not specified the right languages, or you have mixed the languages - without proper compiler flags.
if it's really big, Xcode can spend a lot of time parsing the sources, creating derived data.
if it persists, it's probably a give idea to post more info.