Qsys : Arria V: PCIe Avalon ST source to SGDMA Avalon ST sink interconnect error

As I said I never used that IP, so I can only offer generic help. Is there an example design somewhere that connects the PCIe IP to a DMA? You could see how it's done and reproduce it in your project.

Other than that, the only way I can think of is to make a custom stream component that just connects a sink to a source, but defined as 128-bit symbols on one side and 8-bit symbols on the other. You can connect all the signals one to one, except that the 8-bit side will also need an "empty" signal, 6-bits wide. For the 8-bit sink you can just ignore the empty signal, and for a 8-bit source, set it to all zeroes. Then of course make sure that all your DMA transfers always transfer a number of bytes multiple of 16.