I have downloaded a few different kinds of mirc
There is only one up-to-date kind of mIRC, and that's the one you download from this website. If you download mIRC from anywhere except the mirrors listed on this site, you are potentially risking downloading a hacked, trojan-infected or backdoored executable. Be careful!
There are literally thousands of networks out there. Unless the script is designed specifically to use services on that network, there will be no link between them.
In the unlikely event that a particular script is approved/supported by a network, I would expect a link to it from the network's own website.
When i downloaded it I went to a site and it had all these different links you could download.
That description is much too vague to be useful... it's a bit like saying "yeah, I had this car, four wheels..."
For that one you can be in many channels at once.
All mIRC versions allow you to be in multiple channels. Since version 6.0 (and we're on 6.14 now), you can even be on multiple channels on multiple networks!
The channels that you are in are listed on the right of the screen as are the people you are talking to
The mIRC switchbar does this already, and you decide which side of the screen you want it on. If this is the feature you really like, don't worry and download a fresh clean copy of mIRC, which will do it for you without needing a script. Follow these steps:
1) Open mIRC
2) Press ALt & O to open the options menu
3) Go down to Display
4) Tick the "show switchbar" box
5) Use the box to select where you want the switchbar to appear.
And that's it!
Hope this helps