Hello there. It's an interesting prospect you're presenting here, but if I may, I'd like to suggest some considerations that may help you get what you're looking for.
First of all, your submission policies that you've linked to seem not to consider how most people in digital media comics actually create. Most creators don't break it down into the kind of specific roles that companies like Marvel or DC do; think more along the lines of someone like Moebius or Manara. Most webcomic artists have total control over visual production, from start to finish. Many also write their own stories. It's a rare creator in this day and age that can even afford to bring in anyone else on production that wasn't involved in a story's creation.
But furthermore, you're also assuming a lot, not the least factor of which is the preference for color works instead of black and white or screentoned. Color adds a significant time factor into production in most cases, and it's very easy to make or break an aesthetic with color. I also haven't seen too many letterers involved in the creation of pin-ups, since they're generally supposed to speak for themselves. Maybe you could revisit what you're asking in terms of submissions and adjust them slightly to be more thoughtful to the different roles you're looking to see, as well as more mindful of the actual landscape of creators?
I understand the inspiration of big and familiar companies like Marvel and DC, but smaller projects don't tend to work that way. Especially if you're not paying, it's a lot to ask for someone's love of the medium and telling a story to get them through working with people they don't know and may not work well with. You also don't really have any examples of comics or characters up on your site, so potential collaborators can't see even a vague idea of what you actually have in mind. My suggestion here is to engage people to work on ideas together, to make a cohesive "universe" as you seem to intend. If it's for the love of storytelling and the medium, it'll probably be best to assemble teams based on who even wants to work together and how exactly they do their best work; if you have a writer who constantly micro-manages the artist, a particularly independent artist that likes a lot of creative control isn't going to work well with them. If you have an artist who prefers to do all the art alone, telling them to do pencils while someone else does inks isn't likely to produce the best art, especially if the artists don't understand each other's approaches, which is likely in this situation.
Last but not least of all, just a personal pet peeve: don't tell people you're not going to respond to them. If you're small, you should be friendly and encourage a sense of community. That starts with communication and treating people with a level of respect that you'd like them to reciprocate. In my experience, you're either small enough where you should be replying, and promptly, to submissions, or you're large enough that you can have someone working for you who is dedicated to that task. At the very least, people need acknowledgement that you've received their submissions. If you can't be bothered to let people know, then it's kind of rude to ask for submissions they will never know arrived. Email is less likely to get lost in the post than snail mail, but especially lately it's worth reminding that the internet messes up too.
I hope these suggestions will help you think about what you want exactly from this endeavor, and I do sincerely hope that some of my thoughts and ideas will help you to achieve those goals. It's an admirable thing, to bring people together for the love of creation and telling a good story. If you have a good focus, good organization, and solid plans, it can really make a huge difference. Good luck!
