You could fly them on any bar. Issues you will face are is there sufficient bar throw, does the flagging system provide sufficient movement on the flag line, is the bar big enough and are the line lengths compatible.
A15 needs a lot of bar throw/trim, circa 100cm+. Bigger kites need more throw/trim. The 18m really pushes what is reasonable. My mod improves this though.
For foil kites flagging, a rough rule is half wingspan for fifth line flag and full wing span for front line flag and then maybe a bit more if you are cautious. A15 has a wingspan of between 6.6m and 9.5m depending on the size, check the website. If you limit flag length correctly you can prevent tangles. If you get it too short your kite may remained a bit powered and spin etc, not good. For front line I just let it run free as I want it to work no mater what. I generally don't use a fifth line except on high wind kites. They are good for beginners to give more predictable flagging, you can use them to self land quite well and easy. You can self land on a front line but kite spins more and it is a bit harder on the kite,and can get damaged in high winds( that's from flysurfer experience).
Foil kites like wider bars. Foils are slower and less responsive than LEIs. You use a bigger bar the bigger the kite. I would say a 12m would be good with a 50 or 55cm bar, but 15m would like a 60cm and 18m 65cm.
If lines are not the right length for the kite you will need to make extensions. You are best to make these out of flying line to keep weight and drag down.
One other thing is the way the kite joins the lines. Foils if using the way LEIs attach with pigtails and knots and larksheads then this causes tangles when they can mix with other lines or bridles. This can happen every fucking time you setup or pack down. And worse if you front stall the kite and lines are slack it can even happen in the air, not to mention with slack lines on the water it happens very easy. I surf with my foils I slack lines like you have never seen before. My kites get rolled in waves and have all sorts of things happen. I never have this issue. My setups are a breeze, something I hate about LEIs. Flysurfer a long time ago invented a little metal connector called the ELC, Easy Line Connector. It prevents tangles very well and My A15 came with them, Thanks Pansh, and I have many spares from other kites. The other option is to thread the lines onto the kite like the bridles connect, but this will cause a huge problem of making access to bridle a bigger pain than it already is.