I have a fairly opposing view to Eric's so thought I should post it as a separate answer:
While there may be some spirits that may be okay to mix almost all spirits have distinct differences between brands.
Vodka: if you have plain vodkas, designed to have as little taste as possible, and just add alcohol to fruit juices etc, these will be good to mix. I would not suggest doing this with for example a Stolichnaya, or Bison, or other distinct vodka, or with any flavoured vodkas.
Gin: you really cannot mix gins, as they all have very different flavours from the mix of botanicals. In fact that is the whole point of keeping multiple bottles.
Tequila: Unless you have really cheap tequila (the kind you use for shots and hide the taste with salt/lemon) they all have distinct tastes, and it would be a shame to mix them - they all make different margaritas, for example.
Rum: again, even within the main groups of dark, light, overproof, spiced, navy etc you have a multitude of flavours, so mixing these will dilute the very notes they are famed for.
Bourbon: between bourbons, Kentucky whiskey, Tennessee whiskey and others, there are huge differences in sweetness, smokiness and other notes. These are also a no-no for mixing.
Also, don't even think about doing this with whisky
To answer your specific questions:
- you'll take away the flavours
- it's perfectly safe
- they will taste different, but depending on the mixers and your guests' palates, that may be a good or bad thing
- if you mix equal quantities, the new alcohol level will be the average of the two
Disclosure: I currently have 12 gins, 3 vodkas, 8 tequilas/mezcals/reposados, 6 bourbons, 11 rums and 32 whiskies in my drinks cabinet. They are all so different, mixing them would be a huge waste.