You can install cIOS in any slot above 200.
By convention we reserved these slots for these use :
0-199 : Official Nintendo slots
200+ : homebrew, sorted like that :
202 : DVDX (not used anymore)
220-229 (the "20" range) : Hermes IOS
236 : patched version or IOS36, adding NAND access, and Trucha bug (signature check patch)
249 : default slot for lot of homebrew using cIOS (USBLoaders, Triiforce, Mighty channel, WAD manager, etc.)
250 : used to be a backup (clone) of 249.
254 : Bootmii IOS
256 : BC (official, even if above 200)
257 : MIOS (official, or custom, there's only one slot)
All other slots are free to use, but it's also common to use slot around 249 for Waninkoko and d2x cIOS :
A good range to choose : 240-251
If you install in slot 201 or 233, it works, but it's just strange place to pick because nobody did it.
But try to limit unnecessary used slots, just replace the one you don't need anymore.
Users (and softmod guides) are usually installing in these slots :
249 : default for USBLoaders, d2x based on IOS56.
250 : d2x based on IOS57. slightly slower USB read speed, but best compatibility.
251 : mostly used on vWii, d2x based on IOS58. some games might require a base58, like games with USB2.0 accessories.