Avalon Streaming simple example

That's correct, Avalon-MM even has a concept of symbols. 99.9999999% of the time a symbol will be 8 bits (byte) but the reason why even the memory mapped standard supports non-eight bit bytes is for things like transporting parity or ECC information along with the data.

In general if you are moving data between Avalon-ST and Avalon-MM (i.e. a DMA) then usually you use the same symbol size. If you are moving data between ST and MM pay close attention to symbol ordering because most ST IP has the least significant symbol in the MSBs (some people call this big endian.... but 'network order' is the more appropriate name for it) while MM IP is little endian. Some IP blocks are written without the developer knowing about this which can lead to symbols getting flipped around between the two domains.