I have a chance to get a 7’ Med Moderate Falcon Mansfield Special baitcaster rated 1/4-1/2 I know Moderate action is usually for Crankbaits but would it work for a Whopper Plopper 90 since treble hooks are used? 

It would work. Wouldn't be my first choice but it will work. Especially if you use braid. 

It'll work fine. I use a Lews MHAPC with 30# braid throwing the same ploppers.

I have a medium moderate rod for my treble hook lures.

 

I enjoy it when that is doubled up fighting a 3lb largemouth. 

I’d say definitely especially with braid as has been mentioned. The main complaint I’d have would be when roll casting around targets. I’d much prefer a faster action. Otherwise for general use though you’d be good to go. 

Thanks for the help 

Would it work? Yes

I prefer a MH/M......It gives me better accuracy as I don't have to lob it

 I have that model in the falcon original series...That rod fishes light even for a medium power. If You haven’t handled it, I would.  It has a very light tip.  It may work but It’s better for lighter baits with thinner wire hooks.  The half ounce Max rating is pushing it.