Looking at videos of the 207, there are a blue sponge and 2 white pads sitting vertically at the back of the filter. There are 3 baskets. The bottom one has a black sponge covered with a floss pad; the middle basket has ceramic rings; the top basket has carbon.
The easiest thing , if this were me, would be to empty the cirax into the middle basket then put coarse blue sponges on top of the cirax, cutting them to the shape of the basket so there's a snug fit round the edges. Then the fine blue sponge in the top basket, and again cut up any left over blue sponge and the green sponge to get a snug fit. If there are any gaps between sponge and basket water will go through the gap rather than through the sponge, so a snug fit is important. Looking at videos, the baskets seem quite large compared to Juwel sponges so they should all fit. If there's actually more space than the Juwel sponges can fill, more sponge can be added.
You could run both filters together but think about the media in the 207. You don't need to run carbon full time so you that leaves the top basket empty. Sponges are better biomedia than the ceramic noodles. If I was starting a tank with this filter and didn't have any media from another filter, I'd swap the carbon and noodles with new sponge anyway. You have mature sponge already, so I'd put that in that in a 207.
One final thing, check the dimensions of any external filter to make sure it'll fit in the cabinet. There have been members in the past who bought a filter then found it wouldn't fit.