Can you mix Warhammer 40k armies?

In Open Play, you can combine anything you like. There are no force lists, and you can even ignore points (though you may wish to use Power levels). I've long considered building a Radical Inquisition army with units from every single faction, including Xenos and Chaos.

What is the best army to start with in Warhammer 40k?

Easiest army to paint is probably necrons or tyranids. Necrons you can more or less spray with leadbelcher and you are 90% of the way there. Its up to you how much further you want to go with it. Tyranids have more detail to deal with but can look pretty decent with just base colours and washing.

What edition is Warhammer 40k on?

Warhammer 40,000 8th Edition (also known as Warhammer 41,000: Age of the Emperor to the rumourmongers) is the newest edition of everybody's favorite wargame, which picks things up where the Gathering Storm campaigns (AKA 40k End Times, meaning we're living literally after year 40,999 now) left off, and was released