Just to remind you, 1,000 mg (or 1 g) is the dose where increased incidence of heart disease, stroke, and all cause mortality happened, whether you took calcium as part of a multivitamin or separately. I would lay off calcium. There have been 3 separate studies that confirmed the dangers of supplementing calcium, especially in a large dose like 1g. Check out Chris Kresser's blog.
Now, this has been pretty well publicized now that most doctors are aware of it. The 3 studies that associated calcium supplementation with higher death rates all came out since 2009.
Now, is it less dangerous when you supplement Magnesium, Vitamin D and Vitamin K-2 together? Not really sure but I wouldn't take chances. Supposedly Vitamin K-2 directs where calcium goes (arteries or bones) but I don't believe calcium is useful as a supplement period. I believe there is a bit of "calcium leeching" going on due to the acidity of high flour and high sugar diets and because of gluten sensitivity. Remember that if you suffer from celiac disease, you automatically have osteoporosis because of poor calcium absorption. I think it's the absorption issue that's causing osteoporosis and external calcium supplementation could in mose cases be dangerous. So take your K2, Mg and D3, if you must; these aren't implicated in any studies for their ill effects, at least not yet.
Calcium rich foods: sardines, anchovies, spinach, kale, collard greens.
This is a good question and the corrent answer is hydroxyapatite, especially if it includes some k2 or d3. I wouldn't take any other calcium supplements for reasons that Sataran said.
Looking at amazon, seems like a good low price. Have you taken calcium supplements before? It looks like it combines several different forms. I, personally, have trouble with some of these forms, which have cause major bloating and constipation and related pain. The only calcium I take now is Alive! Bone Formula Max Absorption from Nature's Way, which is also plant based and doesn't cause me any GI distress. However, it is more expensive (about $9 more on Amazon comparing both). Good luck!