I've targeted certain agents, mainly from information on agentquery.com, and checked out all I can find about their tastes before I approach them. Some of the sample query letters they display are puzzling - some want "personal", some want "professionally objective" and some don't tell you! Many of them, however, seem to have a fairly long response time, e.g. 3-6 weeks for responding to an e-mail query of maximum 250 words (and it was difficult to be informative in such a short query, believe me!). Is it considered reasonable to query multiple agents at the same time, or perhaps send to two, wait a couple of weeks, try a couple more etc.? I know my chances of attracting interest are slim, so I don't want inadvertently to annoy anyone. Also, if I have been rejected by a certain agent, can I query someone else in the same office? AND (sorry to be so profuse!) for the ones who say they will only respond if interested, how long should I wait generally before assuming that they were indeed not interested?
My current other question concerns a publisher whose advertised response time is 9-12 months (and yes, they have been around a long time and are no doubt very popular). If I haven't heard from them at all (apart from receipt of submission) and 12 months have passed, how long should I wait before asking politely what the current status of the submission is? (In the past, I've assumed about a month more, but this could be bad form, for all I know.) And is it generally true that silence indicates they haven't yet chucked it, or should I read no meaning into this at all?
On the question of multiple submissions, please see this post. Also, since you're querying in the US, multiple submissions are completely acceptable there.
If an agent in one office has said no, take it as a 'no' for the whole office. Multiple-agent offices are a rarity in Australia so we don't tend to face this issue too often, however in an agency with several agents an individual agent may well pass on queries to other agents in the office if they think it's a better fit for them. If you've had a 'no', you can presume that either hasn't happened or happened and it was a no there too.
If you haven't heard from them within eight weeks - if it's only a 250-word query (geee, wonder if I can get away with asking for that ...) - then move on.
As to the publishing company: you're right, give them a month over the time limit and then make contact. Do not read anything into the silence other than the fact that they haven't responded to you yet. I go silent on submissions for weeks - months - at a time and it's because I haven't read it yet. Everyone gets a lot of submissions. We all make promises about time frames but can only keep them if we have minions to help us. Agencies and publishers aren't that flush with cash, ergo, no minions and no timely response.