gauge 13 strings for a C standard tuning would feel like gauge 11 strings for a E standard tuning in my opinion. That being said, I had the 25.5" scale length for a guitar in mind (eg, Ibanez RG). If that is fine with you, shouldn't be a problem. It would be good gauge for a drop tuning because the your last string would have more definition but your other strings would be stiffer. I would recommend you trying gauge 12 strings for your tuning before jumping to 13.
1 problem is that gauge 13 strings are quite limited. heck, even choosing gauge 12 strings can be quite a pain. fit the strings on your nut first before you can tell whether there is a need to trim it a little more to accommodate the bigger strings.
Btw, which guitar are you referring to? An Ibanez RG or a Yamaha RGX?