Thelecraft: A Classic Minecraft Mod Pack

Should I use the stable or unstable/bleeding version of the mod pack?

Stable. Stable. Not bleeding. Stable. Not unstable. Stable. Not indev. Stable.

What if I'm still using the old launcher?

The old launcher has officially been discontinued and will no longer install the mod pack (although it can still play existing installs for now). You are strongly advised to migrate to the new launcher ASAP so that you can continue to receive updates. An in-place upgrade from previous versions of the mod pack is possible (including migrating save data), but the upgrade method is non-obvious.

Does the new launcher support launching pre-Thelecraft mod pack versions?

Yes, although you need a special access code for the launcher to be able to install them (mostly to avoid unnecessary overpublicization of Ruby's pre-transition identity). If you want the access code, please ask for it. The vast majority of people shouldn't need it though.

Is it safe to uninstall the old launcher once I've started using the new one?

Yes, as long as your save data has been copied or moved out of the old launcher's install folder. You are advised to make 100% sure that you have your save data migrated to the new launcher and that it's definitely in the new launcher's software folder before uninstalling the old launcher. This migration doesn't happen automatically - you have to do it by hand.

Is it possible to migrate from the Exciting Eclair Legacy Edition bundled with Asterion Launcher 2?

Wow, that's a blast from the past. Technically yes, but the in-place upgrade isn't supported from Asterion Launcher 2, because it's extremely old (it was kinda expected everyone had moved to version 3 of the launcher by now). Install Thelecraft from scratch, and you should be able to move your save data to the new installation folder afterwards (you'll have to do this manually though).