I suggest asking this on the tmodloader workshop forums instead of this forum. Terraria's workshop page only houses resource packs (music changes, localization changes, and texture changes), while tmodloader's workshop houses actual mods that add new content to the game.

If you want to download resource packs in THIS workshop and play with them, they will only be client side in multiplayer. Since resource packs only reskin things, this also means other players aren't required to download them in multiplayer if the host has the resource pack enabled while playing.

If you're playing with a actual content mod in multiplayer tmodloader, yes, a player that joins would need to download all of the mods the host is using however the game usually just downloads all of the hosts' mods for you if a player that doesn't have the required mods joins the modded server. If the game doesn't download the mods the host has automatically for the second player for some reason they can just manually download the same mods the host has on the tmodloader workshop and that should work.

For the higher res mods i'm not sure what to suggest (mainly cause i don't think there's a mod that does that), but i hope the other stuff i said helps.

I think that exist a resourse pack todo that