Yes this is a good way to get around it, what do you mean when you say they build extra comb at sides? Is that from the side of the small frame to the side of the Lang frame it's tied to?
How much room between the frames and the base do I need?
Yes ... You need to read up on bee space. Give them space inside the hive of less than about 6.5mm and they will glue it up with propolis... give them a space of more than that and they will build comb in it. Bee space is the amount of room that a bee needs to comfortable pass through.
It's why frames and hive boxes have to match in dimensions as otherwise the bees will build free comb wherever they have sufficient room which makes life difficult when you come to inspect them.
You are going to find it a steep learning curve .. I would recommend that you buy a copy of the Haynes Manual of Beekeeping ... it's a step by step guide through the beekeeping year with lots of information presented in a very readable format - all stuff you need to know. in fact, you really needed to know
before you got your bees ...
Time to set aside Youtube and do some proper research or you will be running behind the bees rather than planning what to do next. Sorry if this sounds a bit harsh, it's not intended to be ... but you cannot imagine how many erstwhile beekeepers give up after the first year - generally because they struggle with the demands of keeping bees without enough knowledge to know what is happening and what has to be done.
The Flow Hive marketing has led a lot of people into keeping bees because it looks so simple, chuck a few bees into a box, turn the tap and out comes the honey. The reality is far removed from the fiction of the markeing videos I'm afraid.
Bees are like any livestock..... would you start to keep a herd of cows without knowing a bit about what you have to do to look after them ? 50,000 stinging insects in a box can be a very challenging situation for you (and if you get it wrong) your neighbours.
This forum is a really hepful place for sorting out the problems we all face from time to time ... but it's no substitute for a bit (indeed - a LOT) of proper research.