The protected batteries will keep dangerous currents from occurring, but you never want to intentionally rely on a battery's protection circuitry. Just because a Li-Ion is protected does not mean it's perfectly safe. IMR batts are typically not protected and can handle high currents, but even with those, the limit can be exceeded when the charge is mismatched enough. They can vent just like the ICO batts, just not as easily. When running Li-Ion cells in parallel, it's best to use them in a fixed pack where the cells cannot be disconnected from eachother. I've read of people running removable Li-Ions in parallel in this forum, but the potential for high equalizing currents adds another element of risk. If you're going to do it, you need to be aware of it.
Switching battery configuration is not safe. Batteries in series do not discharge exactly the same. Every time you switch from series to parallel, you will likely get high equalizing currents. These batteries have very low internal DC resistance and it only takes a tenth of a Volt difference to hit the current limit on an ICO batt, about three tenths of a Volt for an IMR batt. Series batts can often exceed that difference when discharging, especially when one is more worn than the other. That can happen even in a matched set due to normal variations in manufacturing.
