Hello. Don't panic if they try to eat it - fleece is made from pastic bottles and is totally harmless to their digestions. It also is made from fibres that don't clag together when wet - if you scuffed off some loose fibres, wet them and squeezed them together they won't form a 'plug'. So, when eaten, fleece doesn't cause a choking hazard.
You do get obvious poo buildup on them as it is the top layer that catches all the poos. Clearing the poos away is fine, they dry out and are easy to pick up.
I change my fleece every two days, covering it with a layer of cotton sheeting that's easy to change more often if one of my piggies has any kind of problem, eg softer poos from antibiotics.
Under the fleece I have newspaper which soaks up the wee. Fleece doesn't soak up fluid so it will seep down to the newspaper and get caught in there. You could compost the newspaper or throw it out.
I used to use woodshavings, then newspaper on top of cat litter (wood based) but since joining this forum I have moved to fleece. I honestly think my girls prefer it, they like lounging about on soft fleece, and I haven't had any troubles with smell which I thought I might.
Hope this all helps, Sarah