It's fine to use your current mix and supplement with the odd science selective pellets - I do that as well - partly as treats to scatter and partly to boost the protein in the existing mix. The new formula is fine
I agree the Junior rodipet syrian food is better - our hamster didn't like the senior one - too many herbs. I also found they weren't 100% fussed on it so I mixed it 50/50 with the Bunny Dream food and give a few science selective pellets once or twice a week.