i know it seems like a waste, but i suspect you will eventually end up removing the internal filter, just to free up more room, which the smaller the tank is the more it makes a difference. i dont think its that much of a mission to remove it, especially if youve got a bucket or something you can put your fish in for half hour while you do it.
Akasha doesn't have many fish, what she has is small, is only going to add a few more to the tank, knows not to overstock, and most importantly is on a budget that only just stretched to a new tank. She doesn't NEED an external at present, especially not on a 100 litre tank with a few live bearers and corys in.
I have a 180 juwel tank and run both the built in internal and an external eheim. I only added the eheim because I have a lot of fish and some make a lot of waste. Everyone immediately jumps to 'rip out the internal, make more space!' with juwel tanks - if you stock sensibly they're actually perfectly good filters, and I will only ever remove it if I have some larger fish who for some reason desperately need the bit of room it takes up in the corner.
Akasha, rip it out if you want to but I don't think you need to bother at present. Approach this new tank as being more like 100 litres seeing as you can't stock the corner or the gap at the top, and it should be fine with the few more corys you want to add.