first you need a premission if this can be public second if this private and you want your texturepacks mix in one resourse pack you need to make sure you drag all the file for the item-block from one resourse pack to another and delete all the files for this texture in your existing texture pack if you just drag something one to another its not gonna work like this
repeat the folders exacly like in the resoursepack you took this and also you need to make sure you also took the txt coded files for this items or blocks if they exist in the resourse pack any error can crash everything
take every texture the same ways they was placed in resoure pack without placing files from one resoursepack to another and you fine and also some resourse packs you cant combine especially if they have different fronts this can cause some artifacts only if you edit the needed for you fron file you can combine them
hope it helps somehow
remember if you combine textures without the author premission its only need to be your private texturepack do not upload it anywhere
if troubles starten only with new optifine check if you have any shader files in resourse pack its can cause errors and also check all of your textures placement with a new optifine its can sause problems if your textures placed wrong if anything changed

For my texture pack I did not make any of the textures I just put all my favorite texture packs in one