So, you essentially have to have some way to let the light in if you want to build something deep down, I guess? Well hell, they should have something that works better than torches and glowstone, perhaps electrical lighting.
I haven't toyed with 1.8 yet, but the latest version of the texture pack I use includes a zipline mod, whatever that is.
From what I understand, Alba, if they're things that modify the game world, they won't work in multiplayer, but if they're merely client-side like the minimap mod, you should be fine. If they're things that alter the gameplay significantly, unfortunately Minecraft has no way to distinguish them between multiplayer or singleplayer, so they won't let you on multiplayer. That's the disadvantage of having a single profile for both singleplayer and multiplayer. In the future, they could have singleplayer and multiplayer profiles allowing to choose what should be loaded in each profile, but that's not likely to happen until it's out of beta.