1) You don't need to load a script in perform. Once you've loaded it one time, it remains loaded every time you start mIRC unless you unload it.
2) You can run as many scripts as you want to, though if you have a lot of scripts that do cpu-intensive things, you'll notice a performance decrease.
3) How to use a script depends on the script. Check the script's documentation, or look where you got the scripts for how to use them. Sometimes there are triggers like !help, sometimes there are options in one of the right click menus or the Commands menu... it all depends on the script. I'd *guess* that a slap script would allow you to right click on a nick and choose an option on the popup menu for "slap" ... but that's just a guess based on the type of script. The best thing to do is to check any and all documentation for every script you use.
