Simon,
I'll give you a few tips on what you can do with I/RT as far as managing your symbol count.
While IB might limit you to watching 100 symbols per day, I/RT has no such limit....it will allow you to monitor and scan whatever your datafeed (and/or your processor) will allow. DTN IQFeed allows you to monitor 1300 symbols realtime I believe. eSignal has 500 and 1000 symbol limits as I recall. So you can expand list of realtime scanned stocks by changing datafeeds, although I understand that might not be a decision you're willing to consider.
Another option is monitoring 1,000s of symbols on an end-of-day basis, and then trimming that large universe down to a manageable number for realtime scanning. With I/RT's built in Yahoo! screener, and the ability to download historical data, current quotes, and fundamental data from Yahoo!, you can quickly build a large universe of stocks with daily and fundamental data. You can run a daily scan on some criteria to trim it down to 100 or less...mark those symbols for intraday....and then do your realtime scans during the session on that group. You can also use the Yahoo! "current quote" option to grab the current price quickly throughout the day on the larger universe of stocks if that would be of any use to you.
Just a couple suggestions. And you have find more info on the Yahoo! functionality in I/RT at
http://www.linnsoft.com/yahoo/. This is built into every version regardless of your data feed.