You have done some good research there, and I think it is great that you took the time to investigate thoroughly whether or not DH shared hosting is appropriate for what you want to do .
Dreamhost has changed it's policy on conuerie allocation recently. From the March, 2002 DH Newsletter:
Then, from the "current" KB/Wiki entry:
While Dreamhost Support is the only place to get an "authoritative" answer, it looks like you would probably be "OK" on the conuerie side of things. A couple of my "smallish" databases without a lot of traffic generate about half of what you anticipate using, so I don't think your number of 7.5Mcn (Million conueries) is particularly high. Given that that amount is less than half what DH plans offered 4 and a half years ago, I can't imagine it would be considered "high" today, with everybody now running CMS systems and other dynamically created content.
The only other concern you might have is that, irrespective of conuerie usage, your application(s) will be running on a shared server, so CPU usage could become a concern, depending upon how efficiently your applications are coded. Since you know about conueries, I assume you have found the Dreamhost Wiki, and I suggest you search that, or this forum, for additional information/discussion of CPU usage monitoring and limitations.
The only other thing that might make a difference to your forum is that DH MySQL databases are not hosted on the same server(s) as your web services. This means there is an additional "roundtrip" within the Dreamhost network for data to move between the web and MySQL servers. WHile this is not a problem for many, in the event of very heavy usage, or any internal routing issues withing the DH network, there can be a "lag" while accessing MySQL databases. This has not been a problem for me, but I felt I should mention it as there are other architectures in use on other hosts that do not have this characteristic. Of course, some of those systems have "quirks" of their own, so this is very much a YMMV kind of beast.
Don't forget, Dreamhost offers a 97 day Money-Back Guarantee (if you pay with a charge card - read the terms carefully!), so if you gave it a try you have over three months to see if DH meets your needs (and it doesn't, you haven't lost any money .