Should I buy Ram memory DDR3 1066MHz for Mac mini (Mid 2011)?
To be sure you are getting something that will work I suggest shopping with a specialist Mac dealer like OWC, where you will be asked what machine you have and then you can choose from the memory they offer that fits it. They ship to Australia so Spain should be no trouble. Sure you can work it out and get something cheaper but I don't have time for that generally, don't mind supporting a dealer who supports my machine better and if it goes wrong I can say "but you said it would work!" (haven't needed to yet)

Besides I like having as much memory as possible and they offer a 16GB kit. With Xcode, Safari and Pixelmator open I often see more than 8GB in use.

I have only once kept the memory in a Mac when upgrading and in a mini with two memory slots you almost certainly won't. You will want to have two identical memory modules. The nice thing about the mini is that upgrading is so easy, you can get to the memory as soon as you take the bottom cover off.

For your Mac they offer 1333Mhz memory. Looks like it is what it came with also. I would personally not buy 1066MHz because even if it works perfectly it is possibly going to cause the memory bus to run a little slower than it could. RAM is so cheap right now and the wrong RAM can be a really big frustration, why take a chance?