What are the requirements for the people saying the blessings (shevah brachot) under the chuppah? E.g., need they all be male? be shomer Shabbat/observant?
Are there different customs?
Traditionally they've been male and over bar mitzva. (There's been some contemporary discussion about why this is customary, or necessary, but it's the way it's done.)
As long as they can say the words and believe in God, I don't see a problem with someone non-Orthodox doing it. (Just as plenty of synagogues will allow someone not-fully-observant to make a bracha on the torah, or even serve as chazzan.) I've seen someone non-shomer shabbos do one. Some rabbis might bristle at a card-carrying Reform rabbi being given the honors, but run-of-the-mill non-observant Jew? Probably not an issue.
Just to add -- if you're looking for additional visible honors for some non-observant friends or relatives (well, let's assume male ones), you can go with a chuppah that needs people to hold up each pole, and ask people to do that.