Melagras, I follow the KISS principle (as with most things): Keep it Simple, Sista.
I don't drink green smoothies everyday, so I don't concern myself with the oxalate issue, which I believe pertains to spinach, and not kale, collards, or swiss chard. I am not 100 percent certain about that, but I believe I read something of the sort somewhere. As for flavor combos - honestly, I just wing it based on what is in my fridge.
Normally, I use either a frozen banana or 1 cup of frozen mango (I flash freeze my own fruit - I don't purchase fruit that's already frozen). Then, I add only one or two other fruits (1/2 to one cup each - fresh or frozen), plus two big fistfuls of greens, 1-1/4 cups almond milk, and a scoop of either hemp or vanilla flavored vegan protein powder. I like to use mangoes and bananas because they thicken and add sweetness. Also, frozen fruit chills the beverage nicely without dilution. Sometimes, I may add 1/2 to 1 Tbsp of chia seeds, as they add a nutrition boost, and also thicken the mixture. I don't use flax in my smoothies because I get it from other food sources. Occasionally, I may add a tsp of maca, or some other superfood, but I choose just one. If you try to put too much into your smoothie, the flavors overpower one another. So, basic rule of thumb - just two or three fruits, two big fistfuls of greens, milk, protein powder, and sweetener to taste, if needed.
Combos suggestions using any greens of your choice -
Banana/strawberry/orange
Banana blueberry
Banana and/or mango & pineapple (kiwi would go well here too)
Banana and/or mango & peaches (fresh peaches are best)
Mango/Papaya (strawberries would be nice here too)
That's what I can think of off the top of my head. You'll notice I didn't include apples or pears, though they would probably be fine. I typically juice those. If you search the Web, you should be able to find many smoothie flavor combos. I can't stress enough not to add too much "stuff" into one drink. I found that makes a big difference in how pleasurable the smoothie tastes.
HTH! Cheers!!