Using 2 different brands of interconnects?

It's not a problem, especially in your price range, unless you're talking about about mixing within the pair, e.g.: Brand X for the right channel, Brand Z for the left.

There are no problems with using different interconnects on different pieces of gear at all in act in most cases t is preferable.

Please watch out for RCA cables that won’t be good for tube gears but they meant for SS.  It happened to me!  After my guru made a whole bunch of RCA cable for me with good seal of the cable itself and with good plugs it took care of all the interfered noise—gone yesterday...gone. Good luck!

Not just in audio, I always like to mix things. To me, it is so much more fun.

The size of the wires matter more than anything else, as for the material copper is pretty good for sound next is gold after that light or mercury whatever you fancy. Most high end cables are just a gimmicks, remember that sound has reached its limits and all has been discovered so now companies have to make something up to make new big sales.