Was at Big Lots and I found some Monster Garage Cooling System Conditioner made by Zecol. Think its worth using or should I just leave the coolant alone with out any additives?
None at all. The inside of my radiator (and I'm hoping my heater core as well) is sparkly clean. I use the product to extend my coolant an extra season or 2 and to keep the water pump lubed.
Here's a good "conditioner" for you. Drain and fill every two years, it'll leave everything in the best possible "condition".
Absent a statement and a guarantee that a given "conditioner" doesn't change the chemistry of the coolant in an adverse fashion, increase cavitation at the water pump, or reduce corrosion resistance or protection against temperature extremes it really ought not go in. There's certainly no independent nor expert opinion out there to consult. So why bother? For a street car, these are worse "snake oil" than oil additives.
Seriously, with a name like "Monster Garage Cooling System Conditioner made by Zecol"..
If it works as good as RMI25 but costs less it would be worth it. Some testing might be necessary. I wasn't big on additives but there are some that will work wonders in certain circumstances.