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How about... start with all as random, make ones with non-significant SD fixed, check all remaining SD are significant (if not maybe kick those to fixed), one you have a stable model use that as best and then add in each fixed one and see if SD is significant and improves model fit (LR sig), if it does then maybe note it as possible one to include as random. If you have multiple possibles see if they can all go in together and improve model fit and stay sig on SD