Is it okay to use brass inserts in a plastic reverse osmosis pipe?
Here are some that appear to be fully-plastic, and/or plastic + stainless steel

Without knowing what particular fitting you need (1/4 compression to what?), hard to get more specific than that. Full plastic fittings are out there, and really are what you want, as RO &amp; DI water do love to eat anything that will  give them ions. My reading on the above fittings is that the stainless (which is as good a choice as you get in metal, though aluminum may also work well from what I recall of an old lab D.I.W. system) is not in the normally wetted path.

Here is another supplier's nylon/polypro fittings: