Two things:

1/ Intervals: If the switching was instant, you would be making most out of a short interval in theory. This is because, as said, you'd be nimbler to jump on whatever coin is most profitable at any point in time. Unfortunately, miners take time to switch. There's an initialization phase, every gpu starts up, sometimes it takes over a minute to get all your cards hashing. This is inefficient. So you can even this out by forcing your miner to stick around a bit longer per algo. I however do not believe you'd be doing yourself any favour by setting intervals longer than 10-15 minutes because you then lose that flexibility.

2/ Stability: My rig (5x 1080) is crashing several times a day with Nemosminer. I'm talking once an hour on avg. That's not good. On single coin it chugs along perfectly fine without reboots. I haven't yet been able to pinpoint the issue, so I can only *guess* either one instance of ccminer is unstable or the switching causes it.