Skins are very image specific, so always better to have a backup of your image before installing a new skin.

all in a ipk, means it's mipsel+mips32el+mips32el-nf compatible,

ipkg tool to install ipk, is generally called opkg, and if called ipkg in fact it calls the opkg file. So not all images have both, some have only opkg.

Concerning the "opkg install -force-overwrite", the -force-overwrite is as it's wrote, to force the overwrite also if the package would not need an update... but could need it ;o).

TS