If you are talking about the Gods Rituals, use the pronunciations written there, these are crafted perfectly, and precise.
About general workings, I heard to use one style at the time.
Like you use 3 Runes for protection, pronunce all 3 in Gothic, or all 3 in Icelandic.
That's what I heard.
I don't know about mixing these, I never mixed these in my personal workings.
I know different pronunciations have different feels, but allways related to the same Rune.
I wonder about what can happen if instead of doing a triad like Algiz-Algiz-Algiz, I do Algiz-Elgr-Algs.
Do someone know about this?