g++ always backward-compatible with "older" static libraries?

The G++ ABI for C++98 code is backward compatible, all the way back to GCC 3.4

So if you compile and link your final executable with GCC 4.8 you can link to objects and libraries built with anything from GCC 3.4 to 4.8 (but no newer)

The C++11 ABI is the same as the C++98 ABI and the standard library types that are common to both C++98 and C++11 have the same definitions, _{(ignoring GCC 4.7.0 and GCC 4.7.1, which had ABI incompatibilities in std::pair and std::list when using C++11, which have been fixed in 4.7.2 and later versions)} so you can link C++98 and C++11 code together _{(unless the C++11 code was built with GCC 4.7.0 or 4.7.1)}

However some C++11 library types are not stable yet and change between releases, e.g. because they were first shipped before the final C++11 standard and had to be changed to match the final rules. So it's not necessarily safe to mix C++11 code built with GCC 4.6 and C++11 code built with GCC 4.8

For your case, where all the C++11 code is built with GCC 4.8 that will be OK. If you upgrade the compiler you should rebuild all the C++11 code with the newer GCC to be safe. (You don't need to rebuild the C++98/C++03 code)