Couple of ways you can raise humidity - one is to hang wet towels in your tent - but the way I like to make SURE seedlings have as much humidity as they need is to soak a towel and encircle the seedling. This still gives them access to fresh air and yet keeps the air right around them high in humidity...
As for nutes - I know you're growing at least one autoflower ... they don't like ANY nutes for about 2 weeks or so. When you introduce nutes to them, do it slowly - i.e., about 1/4 strength so you can see if they're tolerating them or not. Make sure all water you give your ladies (and nutes, when the time comes) is PH balanced to between 6.0 and 6.5. If you don't have a PH pen, I urge you to make the investment and get one NOW. It will make all the difference in the world to your grow. I like Apera because it's just as good as (if not better than) Blue Lab which, I guess, is considered the "gold standard" and is cheaper to boot.
Don't overwater your seedlings... at this point, all they really need is that humidity!
Good luck!