Right. What you were told was probably refering to Bukkit Plugins. Thouse you can not run with Forge Mods.
SpongeForge and Sponge plugins can run with modpacks. Infact SpongeForge will try and get as much info out of the mods so that plugins can interact with the mod as best as possible.
For example. A shop plugin will be able to sell a mods items. The plugin “ships” (by myself) can use forge blocks as ship blocks.
There are some cases where forge mods and sponge will not get along. This can result in a verity of outcomes from sponge not interacting with the mod to sponge not loading. But try it and find out