I have a novel out with several agents, in partials and fulls. Recently, a small press offered on a different novel of mine. Would it be appropriate to email these agents and tell them that another novel of mine has an offer, even if it's a small press. I'm not looking for them to represent that novel.
Also, I know that low numbers on a self-published book can hurt an author's traditionally published book's first print run and sell through expectations. Is the same true for a previous book published by a small press?
First question: yes, you should email the agents looking at your manuscript to tell them you have an offer on another novel with a small press. If this were a writer I was interested in representing, I'd want to know AND I'd want to look at the contract even if I didn't sell the novel.
I recently looked at a contract for a friend of mine who had an offer from a small press. It was an UTTER disaster. Film rights, world rights, option clause from hell. The works. I think I sent three pages of notes and we crossed our fingers. Fortunately the press was glad to work with her on the terms of the contract and agreed to most of the changes.
Where you can REALLY tie yourself in to a knot is agreeing to a bad contract. Then when you have an agent for your other book, the terms of that first contract can limit what you can sell, to whom, and when.
Second question: Yes. If your sales numbers are on Bookscan, editors will have access to them. If you sell 25 copies of your first novel, your agent is going to need a pretty compelling reason an editor should give you thousands of dollars and expect to sell thousands of copies.
Publishers prefer a blank slate with new authors (less so now than they did ten years ago but it's still largely true.)