What are people thoughts on mixing up different types of yoga? I have been trying Iyengar, Vinyasa Flow, Ashtanga & Hatha classes and they all have their pros and cons.
However I have been told that I really need to pick one style only, concentrate on that and that doing other styles will only hold me back and confuse me.
Maybe its OK to concentrate on one, and just do other classes occasionally for variety?
Variety is the spice of life. Its purpose is to sample and find what suits you so that you may grow. Likewise, too many cooks spoil the stew. Therefore an ongoing sampling might be “nice” or “fun” but it’s not likely to be potent. And, while it is not likely to be potent, it is likely to muck up the waters with inconsistencies and contradictions.
You are, of course, entitled to practice whatever you like however you like. But I believe it’s clear that for most things a focused, laser-like attention and commitment to the doing yields the more profound result - and Patanjali backs me up:-)
It depends on your goal you want to achieve from yoga and how you feel in each of them. All roads lead to same goal in yoga.
As all the styles you mentioned are asana based, trying them does not look to me too much “mess”. Can you figure out what could be practicing Kundalini, Mantra, Hatha, Kriya and Raja?
My advice: trust your intuition.
I think you should do what feels right for you. I’ve practiced more styles of yoga than I can count and found benefits in every class. Yoga is a personal journey.
I would advice you to find out what different yogas actually are. You know, get a basic overall understanding of the various kinds and systems of yoga so that you can analyse and figure out where different practices aim at. What you list are all physical oriented practices. Mixing them is not a problem as there are no initiations (or mantras) involved, but of course you might overdo physical practice. It is also good to stick with some form of practice for some while to get some exp how it works for you. Perhaps you feel at the first session whether you like it or not, or if that’s something you’re looking for.
When it comes to other than physically oriented yogas then it is a different ball game. Again, study and figure out what you’re dealing with. There are many open discussion forums in the web and other sources where you can find out what you’re dealing with. Use Google, you know 
Please look for quality and not quantity. Even doing basic simple yoga has enough potential to give you all results…
I love to read different books and articles this YouTube about transformation was great!