How much speed should I give to MixMamo? Tyring to outurn Modest Heatran seems risky, as most are Scarfed nowdays, and I also want Blizzard to have more power.
Mix Mamo is a tough one I find, but there are some useful numbers that you might want to try out. I must stress that It does depend a lot on the make-up of your team. You have enough power with Blizzard to 2HKO most Skarmory, but you could chuck in a few more to ensure the 2HKO on clever Skarmory who run a few SpD EVs, you need 72 SpA EVs to 2HKO the Skarmory in Obi's Stall team with Stealth Rock and Leftovers, assuming you hit it on the switch in.
Moving on to Speed, when I ran Mix Mamo, I ran + Speed, as most Adamant Lucario don't expect it, and are outsped. This needs a Hasty / Naive (Naive is probably best) nature and 236 Speed Evs to accomplish. You also outspeed Jolly Gyarados, Max Speed Metagross and Timid Heatran, if you want number specific to those three ask or use the Speed tiers.
If you don't like the attack drop you could run:
252 EVs - Draw with other Neutral / Max Speed Mamo
232 EVs - Beats Modest Heatran
200 EVs - The absolute minimum (or possibly 204) 246 / 247 is a good number because it outspeeds Jolly Tyranitar and most things that are deigned to outspeed it.