Hiya
You're ok to purchase Creative nail polishes without being trained. With regard to working for someone who uses Creative products - I guess if your employer knows you're not Creative trained then I guess from that point it's up to you and them. However, you won't get the best from the product and there will also be a good possibility that you will run into problems!
If you want to use CND L&P why not do their conversion course. You get a really good value kit plus you also get the knowledge to back it up. Of course from there you can also go on to do their infamous Master classes
Adele