The Skirmish are indeed pretty easy - there is no difficulty level. You can handicap yourself by not buying certain units, or, vs your wife, you can decide not to use all your money (say, you're only allowed to spend 80% of the money you make; this makes it tough on the bookkeeping, but personally I'd do it immediately if my girlfriend wanted this as a condition to play me
)
As for the online games... it's always pretty hard to find games. At least it is for me (I live in Europe, which means I typically don't have the same free hours as Americans). The online game comes with its own permanent ladder.