Hey I'm trying to get forge to work with Minecraft 1.2.5 using the new launcher and the new installer, but the older versions don't have an installer.

is this even possible or do I have to download the old version of Minecraft and load forge onto that?

Its possible, and you need to manually install it.

We do not support old version, there is honestly no legitimate reason for you to still be using 1.2.5

for up to 1.5 you can use multimc

1.2.5 has zepplen which by the way can't be updated because entity bounding boxes flicker on client side! (Lex fix this please!)

We're not gunna update 1.2.5 shit. Have the mods update. Nobody here cares about it.

If there is a bug, then debug it and submit a pr to fix it.