I thought the same way when I purchased my skimmer. I didn't want to buy a skimmer twice.
Mine isn't mainstream and it isn't either in-sump or hang-on. It stands on it's own. I've had it running for 5 years now, and have yet to replace a part. It's non-adjustable, which is nice. Easy to clean.
It's too big for your present 100g, but if you're considering upsizing in the future, it may be worth a look.
(Aquamedic)
Mitch