Make sure whatever soap you use is not anit-bacterial or it will kill off all your beneficial bacteria. It's fine to mix them together but you need to make sure that the pH of it is 7.0 and to mist in cooler temperatures.
I personally stopped applying neem as a foliar feed all together and apply to the roots instead. The plant still intakes it and there's not the risk of leaf damage from foliar spray. But if you're confident in your spraying abilities then I wouldn't worry about the mix of the two together at all.
Oh, apparently the beneficial bacteria in soil really enjoy a good neem drench as well. Plus, you can apply it during flower without worry about molding buds from spray.
I love neem oil, I could really stand on a pulpit and give it praises, but I'll sum it up like this... You can apply neem every 3-4 weeks and probably never deal with a pest, or you can hope an pray you don't get an infestation and maybe end up having to apply something like pyrethrin every 3-4 days to take down a fast breeding pest infestation. I haven't had a single issue with pests sense I started using neem; in fact the only time I see them is when I find their dead remains on the tent floor.
One note about neem though, is that it can effect beneficial insects so use it carefully around pollen bearing plants, otherwise it can mess up the honey bees.