Plugins are written for a server, using the bukkit API (if you're writing it for a bukkit server that is)
Mods are written (usually) for single player using the Minecraft API but alot of minecraft mods have been written with servers in mind.
I believe Tekkit is just all those Mods (IC2, BC2, RedPower) linked together with some programming magic from the technic team, so you wouldn't write a Mod for Tekkit you would write a mod for minecraft.
Finally you need the technic team to actually like it and want to put it in Tekkit, it's not like you can just drop your Mod into a folder and it'll suddenly work