As the rules stand right now mixing is sub optimal since you can only use any given stratagem once per phase on 1 unit. Mixing will mess up target selection and buffing. For example, if you are using Alpha Legion, the only legion that can field semi competitive havocs imo, then you want 4 of the same weapon double shooting every turn and protected with concealment so they can keep dealing damage for multiple turns. If you mix the weapons you will do 2 jobs badly rather than 1 job well.
For the future you got a few options.
1. Don't buy an emperordamned thing until GW proves they can write a good CSM codex. Hasn't happened in a decade and some trust needs to be built up before GW is given any latitude.
2. Buy and paint your models but magnetize everything. This is a hobby PITA but it saves some money.
3. Buy lots of havocs so you have enough models to field multiple units of any loadout.
4. Don't buy havocs. Unless they go to 3w or can blob up to 10 models in a unit i dont see how they could be good. They just wont have any ability to stick around so any points investment in them is only ever going to give you 1 turn of shooting per unit.
For the record I picked the dumbest least disciplined of these options and can field 10... 4 lascannons and 4 missile launchers. I even made them the worst possible legion, World Eaters.