Just to clarify, guinea pigs must be kept in pairs. Even aggressive piggies should at least have some interaction with another of their species between cage bars.
1. As long as it’s secure, yes. I would not use a fabric playpen as this would be easy to chew out of; I’d opt for a C&C playpen.
2. I always recommend providing both so your piggies can choose themselves.
3. It really is up to you, one isn’t particularly better than the other. I opt for fleece because I find it to be cheaper and less messy, but then I have to make extra pee pads to spot clean instead of just removing dirty bedding. Fleece also has very little odor control. If you wanted to use loose bedding, you only need an inch or two—guinea pigs don’t require the ability to burrow to be adequately enriched.
4. Depends on your cage’s size. If it’s closer to the minimum (4x2’) I’d shoot for a couple hours a day but if it’s more like 6x4’ for a pair ~15 minutes every few days is fine.