I have a couple of futaba 5kg servos think there 3150. Would they be ok to use on a 1/6 scale cougar? Or even on the in boards

Not really!. It would depend on the control horn set up and leverage. The Cougar has a huge set of flaps. The Derjet Cougar fuselage flaps have a vertical pushrod with a large leverage, so you could use them there.
Flaps should use Digital servos, you need holding power to resist the push back force and non digital servos on most flap set ups are too in consistant

Dw

There digitals, there the 3152,? What servos did u use in ur cougar Dave?

On the wing flaps JR 8711 and on the fuselage flaps JR 8411

regards

Dave