No I think you're right switching back and forth isn't generally a good idea. It's also worth remembering that provided your piggies are getting fresh hay and veg with vit c in it each day, nuggets aren't an essential at all anyway. They are a great way, I think, of making sure piggies do get certain essential nutrients.
I would view it as you feed the mint ones but some days you don't feed nuggets but give a different extra treat instead - and that treat just happens to be the Blackcurrant nuggets!
But, please, don't feel pressured to make any changes because of what I say. Everyone's situation is different and things that work for some piggies might not work for others too. I've fed the blackcurrant nuggets in the past as the "regular" nuggets when I found a really cheap deal on them and I wasn't able to buy the more expensive nuggets.