ghosted, FOTSGreg made some very valid points and I think you overreacted to the questions posed.
You're asking for published writers to complete a novella. That's fine. But there are only two types of anthologies out there: a) those driven by the publisher---who hires authors to fill a slot based on their already published works, and b) those driven by one or more authors. As far as I can determine, you've created a project that is like b). I've been involved in both kinds of anthos and it's the issues you HAVEN'T discussed here that have me concerned, such as:
1. Submission and Sale. You've stated you're going to FIRST write the novellas and THEN try to sell them to a publisher. Bad plan for an antho, unless you're going through a house where one of the authors is already published. I worry that the publishers you've contacted all want to see the written novellas--especially when they're themed. It might be that the editor will have a different vision for the theme you've contemplated, and that will spell problems for the authors in rewriting what they've already done. Published authors have other commitments and plan their schedule to fit in small projects because they DON'T have many problems. With the novellas already written, that will be a hard sell unless the editor's really on-board with your idea. Most anthos that are author driven are sold by query and proposal with a simple statement that: "Here are the authors already signed on, and here's their level of sales." I just signed on for an antho absolutely brimming with significant bestsellers (NYT and USAT all) in UF, F, and Horror, and it struggled to find a home. It'll be published, but by a Canadian SF press, rather than a NY one.
2. Contracts and Royalties.The legal complications of being the "editor" of an anthology will alarm you once you sit down to think about it. Unless the publisher agrees to provide separate contracts to each person, it'll be the point person under contract. The point person, in turn, will have to make sure that each author meets their obligations (instead of the publisher doing it) and the point person will receive all the money and have to distribute it. That money will have to be doled out through the normal course of business---a trust fund set up to hold an advance and future royalties, checks mailed to the authors, 1099s to the authors at tax time and the requisite tax filings for the business. It's far, FAR better for the publisher to send separate contracts and let them deal with the hassle, but it's a struggle to get them to do it, because it's easier to work with one person and let them deal with the pain. Hopefully, gillhoughly will wander by and notice this thread. S/he's been involved in numerous anthos, both as contributor and as editor. I'm sure s/he'll have solid input about the things you need to consider.
3. Credit. It can be a p*ssing match with pubbed authors when it comes to the front cover. Generally, it's the biggest seller who gets the main credit on the cover and that can change in an instant if someone hits a list. Yes, it's only 3 novellas, which is easier, but the publisher is going to want to push the person who will SELL the antho to the public. Are you okay if that's not you? If your name, in fact, doesn't appear on the cover at all?
4. Editing. Are you the person who will actually edit the works of the other writers? If they're published, every work produced must meet a minimum standard of quality. Do you have those skills, or will you be relying on a publisher's editor to make corrections to the theme and to the stories? Keep in mind that the underlying concept of your idea might change in a heartbeat if you remove yourself from the editorial mix. Just something to keep in mind.
These aren't questions to answer to only "those interested." They're required to be answered to MAKE someone interested. Any publishing contract is a serious consideration. Please don't make light of the issues when discussing them with people who would be looking to add this anothology to their publishing credits.