Many spring mixes have a lot of spinach and other things that need to be fed in very little quantities, so not all of them are okay to be fed on a daily basis. I would probably introduce spring mix over a week by mixing it in their normal veggies and slowly increasing the amount of spring mix. It depends a lot on what specific veggies are in the mix, but I’d probably only recommend spring mix be 1/4- 1/3 of a rabbit’s fresh veggies. If it’s a really good brand with a lot of variety, you might get away with making it be half of the daily veggie portion.

Spring mix is fine, but you’ll want to check the ingredients. If it contains things your bunny hasn’t had before, then you will want to introduce it slowly to make sure everything agrees with them. Baby spinach can be found in some mixes, as shasbun pointed out. If it’s just a couple of leaves mixed in, then it isn’t a big deal for them to have it every day. There is enough variety in there that it “evens things out”. I would watch out for mixes with a high percentage of spinach though.

If it’s a good mix, then you could probably go half and half.

I do 100% spring mix when I am busier as it gives a mix with out me having to prep anything or forage. I think they a great easy way to get variety of greens and sometimes they have herbs too. Dorian doesn’t like spinach so he picks those out, sometimes tosses them out of his bowl, so my biggest issue there is a little mess. Still as LBJ10 said there is usually so little in there the few leaves just balance out and I have found a few mixes with none.  If you are worried you can always pick them out and eat them yourself.