I mean, technically you can S-Support anyone if you just want the child units, but if you care about min-maxing and optimization and all that then you might want to carefully consider who you pair with who so the child unit can get the best stats and skills. Child Units inherit the last skill currently equipped to both parents. So, if Kaze S-Supports Felicia, and Kaze is a lv. 5 Master Ninja and Felicia is a Lv. 15 Maid they will pass Lethality and Tomebreaker onto Midori. By using the Equip skills menu you can manipulate the order of your skills so you can make sure you pass on the most beneficial skill to a child.
But, again, you don't have to do any of that if you don't really care about gay number nerd shit.
As for best 3DS FE game, I kinda wanna go with Awakening. Echoes has great art, story presentation, voice acting all that, it's mostly just the maps and gameplay that are tiresome. Sure, Awakening maps are all rout maps on big open fields with nothing interesting going on, but I'd take that over Echoes maps where you trudge through poison swamps fighting a never-ending legion of Mogalls.