in my opinion - Rico M2, then Rico M3. after that - upgrade Viking M1-M2-M3. Then Paint ("Pyton" is best choice as first paint for ricoviking). Dict m3 is good for Railgun|Isida, and not suitable for Rico. If you prefer common battles, combos "Viking + Thunder" or "Viking + Rico" - is the best choice.

paint - no, my opinion - hornet m2, then freeze m1-m2, after that - hornet m3. after that - freeze m3, at last - paint (for example prodigy)

m2 thunder is a best choice. Because in a fututre you will upgrade it to m3. For this moment isida is useless. railgun m3 is worse then m3 thunder. So, do not spend crystals on m3 railgun and invest them in a future - thunder m3.