I really dont know if forge is compatible with old launcher since my computer is weird. But, when I tried to install Forge for 1.12, it didnt work. It said these libraries failed to download. Try again.
org.ow2.asm:asm-all:5.2
com.typesafe.akka:akka-actor_2.11:2.3.3
com.typesafe:config:1.2.1
org.scala-lang:scala-actors-migration_2.11:1.1.0
org.scala-lang:scala-compiler:2.11.1
org.scala-lang.plugins:scala-continuations-library_2.11:1.0.2
org.scala-lang.plugins:scala-continuations-plugin_2.11.1:1.0.2
org.scala-lang:scala-library:2.11.1
org.scala-lang:scala-parser-combinators_2.11:1.0.1
org.scala_lang"sca;a_swing_2.11:1.0.1
org.scala-lang:scala-xml_2.11:1.0.2
net.minecraftforge:MercuriusUpdater:1.12
Please help me, Thanks!