In tournaments are you allowed to custom cosmetics like custom Space Marine shoulder Pads and heads, as long as the rest of the figure, weapons and wargear are all official. I found a website that makes custom parts I wish to use with my Marines but I wasn't sure if they were legal in tournaments Stuff like this, this, this, and this
For cosmetic stuff like this, you should be ok. The ITC rules certainly don't make any distinction about things like that, as long as it is clear what the model represents. Other tournaments may have differing rules - for example the NOVA open tournament states that if your models are different from the standard, you cannot gain any unfair advantages (for example, making a model taller to give it better line of sight), but otherwise states that as long as your models are clear on what they represent, you should be fine.
Generally speaking, modifying or converting models is a big part of the hobby, and most tourneys accept and embrace it - with some even having awards for best converted model or the like. However, it is always in your best interest to read the rules for any tournament you may want to enter and, if you have any questions, contact the organizer to ensure you'll be compliant.
It depends largely on your Tournament Organiser, as there are no 'official' GW run tournaments any more. Most places I have seen are fine so long as the model is WYSIWYG (What You See Is What You Get) - which means that it's easily recognisable for what it is and what it's equipped with (i.e. customised shoulder pads/ head swaps etc are fine, but don't try and pass a lasgun off as meltagun).
The big thing to ask yourself is this: Will there be any confusion from my opponent on what my models are, and what they are capable of?
If in doubt, get in touch with your TO, but I would be very surprised if they turned you down.