Keeping languages separated is always going to be challenging, especially when you're talking about closely related languages like the Romance set.
We would do the following to try and keep them as separated as we can:
- Pay SERIOUS attention to pronunciation and ear training. Having very different sound-sets in your head is your first defense against language mixing.
- Never re-use images on anki cards across languages.
- Never re-use mnemonics on anki cards across languages.
- Never study related languages at the same time.
- Give each language at least a year to really solidify.
Beyond that, it's just a matter of getting lots of exposure, lots of sentence study, and hard work. You ultimately save time when learning related languages; you'll progress faster...but you definitely will pick up mistakes that are specifically caused by mixing, and the only thing you can do to fight them is more exposure.