I really need help with custom rss feeds

I could really use some help here. I need to have various RSS feeds for various categories that display different numbers of items. For example, I want the main feed to list all posts in all categories. Easy enough, I can change the RSS setting to “999” or something. I also know how to get the feeds for the different categories like so:

http://example.com/wp-rss2.php?cat=42

or, since I use clean permalinks, like this:

http://example.com/category/tech/feed

That all works fine. What I need is to be able to have these category feeds show different number of items, so the “tech” category’s feed would only list 5 items while the “web” category’s feed might list the last 10 items.

I think this should be doable by copying the wp-rss2.php file and customizing it. One way might be to edit it so the number of items can be passed along via the url, like so:

http://example.com/wp-rss2.php?cat=42?items=5

(One issue here is if one uses clean permalinks — how would that be passed along?)

Another way would be to have separate wp-rss2.php files, one for each category, with the category and number of items hardcoded into it. I would be fine with this, so I could have:

wp-rss2-tech.php that lists the last 5 items in “tech” category and

wp-rss2-web.php that lists the last 10 items in “web” category.

I think this should work, but I can’t program php worth a lick. Can some kind soul either point me to another solution or, tell me how to edit the wp-rss2.php file so I can get these results? I would be really, really grateful and would send loads of good karma thoughts your way.

Thanks and cheers!