If your reusable nappy does not have a stay dry layer, you can, if you choose to, use nappy liners to catch any solids and flush them down the toilet. Paper liners do not keep babies dry, they just catch the solids. Stay dry liners keep baby dry and catch the solids.
No, although they can be very helpful in certain circumstances. I found using nappy liners completely useless with newborn breastfed poo nappies. However, for old children they were very useful.
Should I choose Disposable or Fleece for my system?
Disposable nappy liners help you on your cloth journey and can make the job of using reusable nappies a lot easier. You do not have to use paper liners, it's a personal choice. Fleece liners will work just as well and are reusable. However, you do not need to use fleece if you have a stay dry layer built into the nappy. When potty training, we recommend removing any fleece nappy liners from the nappy a few weeks in advance.
Fleece nappy liners provide a stay dry layer, and it helps if you remove this for potty training, so your child can begin to feel wet, and connect the cause and effect dots! Not all disposable liners can be flushed, check with the manufacturer! You can use any brand liners with any brand nappy, eg if you have bambino mio nappies, you don't need to use bambino mio liners, you can use little lamb ones.
Biodegradable, not flushable
Note, most liners now tell you to avoid flushing them down the toilet.