I'm the same as squidgernetball I used to have a mix in my kit but I stock a particular brand in the salon so only use this when booked with make up. There's so many pro only brands to use that aren't stocked in department stores, have a look at PAM or Le MAQ PRO as used on the new bond movie! X
I'm just started to work and I have a mix, not retailing yet but planning to do so in the future... It's very difficult to stick just to one brand in the beginning of your job, because you haven't tried a lot of different cosmetics yet and cannot decide which is the best, so I think that you should try many different cosmetics first until you will find a product that you think is the best and then you can decide whether to stick to one brand or to still having a mix. But I cannot see the problem if you like products from different brands why you cannot have a mix, I think that the most important thing is that the products are all of good quality and you can do your job well with them!
I have a total mix in my kit, try and use pro and high end brands but also do use some drug store items that are classics.
Morphe brushes eyeshadow palettes are fantastic when starting out, excellent prices but they are sold out everywhere at the moment!!
My tip when starting out is get your pro discounts in place from nars, illamasqua etc. also try the products on yourself/ truthful friends and then decide whether to invest (thats mainly on foundation I guess) as I could (and I have!!) have wasted so much money on foundations which weren't all that!