Forge and Fabric are incompatible, but you can use Fabric/Forge mod with Vanilla Datapack.
Optifine is a alternative graphic engine for Minecraft that most of the shaders are made for.
Optifine is not compatible with fabric, but can be load in to Vanilla Minecraft or Forge Minecraft.
Cloudron support just Vanilla Datapack, not any other mod loader (forge or fabric) but been Optifine Client-Only it will work.
But if u wont to add mod that you find on Curse/Twitch you need to w8 later September we will release some version of minecraft apps for cloudron that support Forge/Fabric mod.
@scooke Optifine is a client-side mod, just install it on your computer.
However, to answer the question about mods in general on Java server, you will need to recompile the app afaik. You may be able to install some mods if the app has the server.jar file in the /app/data dir.
As for configuration changes, you must restart the app after changing the config (easiest way).